Subcutaneous electrocardiography monitor configured for test-based data compression

ABSTRACT

A subcutaneous and cutaneous electrocardiography monitor configured for self-optimizing ECG data compression is provided. The monitors include a housing, an electrocardiographic front end circuit, a memory, and a micro-controller configured to: obtain a series of electrode voltage values based on the sensed electrocardiographic signals; use a plurality of selection schemes to choose one or more of a plurality of compression algorithms associated with each of the selection scheme for testing; test the selected compression algorithms including applying the compression algorithms chosen using each of the selection schemes to a segment of the electrode voltage series; analyze results of the testing; select one or more compression algorithms chosen using one of the selection schemes for compressing at least a portion of the electrode voltage series based on the analysis; obtain a compression of at least the portion of the electrode voltage series; and store the compression within the memory.

FIELD

This application relates in general to electrocardiographic monitoringand, in particular, to a subcutaneous electrocardiography monitorrecorder configured for test-based data compression.

BACKGROUND

The first electrocardiogram (ECG) was invented by a Dutch physiologist,Willem Einthoven, in 1903, who used a string galvanometer to measure theelectrical activity of the heart. Generations of physicians around theworld have since used ECGs, in various forms, to diagnose heart problemsand other potential medical concerns. Although the basic principlesunderlying Dr. Einthoven's original work, including his naming ofvarious waveform deflections (Einthoven's triangle), are stillapplicable today, ECG machines have evolved from his original three-leadECG, to ECGs with unipolar leads connected to a central referenceterminal starting in 1934, to augmented unipolar leads beginning in1942, and finally to the 12-lead ECG standardized by the American HeartAssociation in 1954 and still in use today. Further advances inportability and computerized interpretation have been made, yet theelectronic design of the ECG recording apparatuses has remainedfundamentally the same for much of the past 40 years.

An ECG measures the electrical signals emitted by the heart as generatedby the propagation of the action potentials that trigger depolarizationof heart fibers. Physiologically, transmembrane ionic currents aregenerated within the heart during cardiac activation and recoverysequences. Cardiac depolarization originates high in the right atrium inthe sinoatrial (SA) node before spreading leftward towards the leftatrium and inferiorly towards the atrioventricular (AV) node. After adelay occasioned by the AV node, the depolarization impulse transits theBundle of His and moves into the right and left bundle branches andPurkinje fibers to activate the right and left ventricles.

During each cardiac cycle, the ionic currents create an electrical fieldin and around the heart that can be detected by ECG electrodes placed onthe skin. Cardiac electrical activity can then be visually representedin an ECG trace in PQRSTU-waveforms. The P-wave represents atrialelectrical activity, and the QRSTU components represent ventricularelectrical activity. Specifically, a P-wave represents atrialdepolarization, which causes atrial contraction.

P-wave analysis based on ECG monitoring is critical to accurate cardiacrhythm diagnosis and focuses on localizing the sites of origin andpathways of arrhythmic conditions. P-wave analysis is also used in thediagnosis of other medical disorders, including blood chemistryimbalance. Cardiac arrhythmias are defined by the morphology of P-wavesand their relationship to QRS intervals. For instance, atrialfibrillation (AF), an abnormally rapid heart rhythm, can be confirmed byan absence of P-waves and an irregular ventricular rate. Similarly,sinoatrial block can be characterized by a delay in the onset ofP-waves, while junctional rhythm, an abnormal heart rhythm resultingfrom impulses coming from a locus of tissue in the area of the AV node,usually presents without P-waves or with inverted P-waves. Theamplitudes of P-waves are also valuable for diagnosis. The presence ofbroad, notched P-waves can indicate left atrial enlargement. Conversely,the presence of tall, peaked P-waves can indicate right atrialenlargement. Finally, P-waves with increased amplitude can indicatehypokalemia, caused by low blood potassium, whereas P-waves withdecreased amplitude can indicate hyperkalemia, caused by elevated bloodpotassium.

Cardiac rhythm disorders may present with lightheadedness, fainting,chest pain, hypoxia, syncope, palpitations, and congestive heart failure(CHF), yet rhythm disorders are often sporadic in occurrence and may notshow up in-clinic during a conventional 12-second ECG. Continuous ECGmonitoring with P-wave-centric action potential acquisition over anextended period is more apt to capture sporadic cardiac events. However,recording sufficient ECG and related physiological data over an extendedperiod remains a significant challenge, despite an over 40-year historyof ambulatory ECG monitoring efforts combined with no appreciableimprovement in P-wave acquisition techniques since Dr. Einthoven'soriginal pioneering work over a 110 years ago.

Electrocardiographic monitoring over an extended period provides aphysician with the kinds of data essential to identifying the underlyingcause of sporadic cardiac conditions, especially rhythm disorders, andother physiological events of potential concern. A 30-day observationperiod is considered the “gold standard” of monitoring, yet a 14-dayobservation period is currently pitched as being achievable byconventional ECG monitoring approaches. Realizing a 30-day observationperiod has proven unworkable with existing ECG monitoring systems, whichare arduous to employ; cumbersome, uncomfortable and not user-friendlyto the patient; and costly to manufacture and deploy. Still, if apatient's ECG could be recorded in an ambulatory setting over aprolonged time periods, particularly for more than 14 days, therebyallowing the patient to engage in activities of daily living, thechances of acquiring meaningful medical information and capturing anabnormal event while the patient is engaged in normal activities aregreatly improved.

The location of the atria and their low amplitude, low frequency contentelectrical signals make P-waves difficult to sense, particularly throughambulatory ECG monitoring. The atria are located posteriorly within thechest, and their physical distance from the skin surface adverselyaffects current strength and signal fidelity. Cardiac electricalpotentials measured dermally have an amplitude of only one-percent ofthe amplitude of transmembrane electrical potentials. The distancebetween the heart and ECG electrodes reduces the magnitude of electricalpotentials in proportion to the square of change in distance, whichcompounds the problem of sensing low amplitude P-waves. Moreover, thetissues and structures that lie between the activation regions withinthe heart and the body's surface alter the cardiac electrical field dueto changes in the electrical resistivity of adjacent tissues. Thus,surface electrical potentials, when even capable of being accuratelydetected, are smoothed over in aspect and bear only a general spatialrelationship to actual underlying cardiac events, thereby complicatingdiagnosis. Conventional 12-lead ECGs attempt to compensate for weakP-wave signals by monitoring the heart from multiple perspectives andangles, while conventional ambulatory ECGs primarily focus on monitoringhigher amplitude ventricular activity that can be readily sensed. Bothapproaches are unsatisfactory with respect to the P-wave and theaccurate, medically actionable diagnosis of the myriad cardiac rhythmdisorders that exist.

Additionally, maintaining continual contact between ECG electrodes andthe skin after a day or two of ambulatory ECG monitoring has been aproblem. Time, dirt, moisture, and other environmental contaminants, aswell as perspiration, skin oil, and dead skin cells from the patient'sbody, can get between an ECG electrode's non-conductive adhesive and theskin's surface. These factors adversely affect electrode adhesion andthe quality of cardiac signal recordings. Furthermore, the physicalmovements of the patient and their clothing impart variouscompressional, tensile, bending, and torsional forces on the contactpoint of an ECG electrode, especially over long recording times, and aninflexibly fastened ECG electrode will be prone to becoming dislodged.Moreover, dislodgment may occur unbeknownst to the patient, making theECG recordings worthless. Further, some patients may have skin that issusceptible to itching or irritation, and the wearing of ECG electrodescan aggravate such skin conditions. Thus, a patient may want or need toperiodically remove or replace ECG electrodes during a long-term ECGmonitoring period, whether to replace a dislodged electrode, reestablishbetter adhesion, alleviate itching or irritation, allow for cleansing ofthe skin, allow for showering and exercise, or for other purpose. Suchreplacement or slight alteration in electrode location actuallyfacilitates the goal of recording the ECG signal for long periods oftime.

Conventionally, multi-week or multi-month monitoring can be performed byimplantable ECG monitors, such as the Reveal LINQ insertable cardiacmonitor, manufactured by Medtronic, Inc., Minneapolis, Minn. Thismonitor can detect and record paroxysmal or asymptomatic arrhythmias forup to three years. However, like all forms of implantable medical device(IMD), use of this monitor requires invasive surgical implantation,which significantly increases costs; requires ongoing follow up by aphysician throughout the period of implantation; requires specializedequipment to retrieve monitoring data; and carries complicationsattendant to all surgery, including risks of infection, injury or death.

Holter monitors are widely used for extended ECG monitoring. Typically,they are often used for only 24-48 hours. A typical Holter monitor is awearable and portable version of an ECG that include cables for eachelectrode placed on the skin and a separate battery-powered ECGrecorder. The leads are placed in the anterior thoracic region in amanner similar to what is done with an in-clinic standard ECG machineusing electrode locations that are not specifically intended for optimalP-wave capture. The duration of monitoring depends on the sensing andstorage capabilities of the monitor. A “looping” Holter (or event)monitor can operate for a longer period of time by overwriting older ECGtracings, thence “recycling” storage in favor of extended operation, yetat the risk of losing event data. Although capable of extended ECGmonitoring, Holter monitors are cumbersome, expensive and typically onlyavailable by medical prescription, which limits their usability.Further, the skill required to properly place the electrodes on thepatient's chest precludes a patient from replacing or removing thesensing leads and usually involves moving the patient from the physicianoffice to a specialized center within the hospital or clinic.

U.S. Pat. No. 8,460,189, to Libbus et al. (“Libbus”) discloses anadherent wearable cardiac monitor that includes at least two measurementelectrodes and an accelerometer. The device includes a reusableelectronics module and a disposable adherent patch that includes theelectrodes. ECG monitoring can be conducted using multiple disposablepatches adhered to different locations on the patient's body. The deviceincludes a processor configured to control collection and transmissionof data from ECG circuitry, including generating and processing of ECGsignals and data acquired from two or more electrodes. The ECG circuitrycan be coupled to the electrodes in many ways to define an ECG vector,and the orientation of the ECG vector can be determined in response tothe polarity of the measurement electrodes and orientation of theelectrode measurement axis. The accelerometer can be used to determinethe orientation of the measurement electrodes in each of the locations.The ECG signals measured at different locations can be rotated based onthe accelerometer data to modify amplitude and direction of the ECGfeatures to approximate a standard ECG vector. The signals recorded atdifferent locations can be combined by summing a scaled version of eachsignal. Libbus further discloses that inner ECG electrodes may bepositioned near outer electrodes to increase the voltage of measured ECGsignals. However, Libbus treats ECG signal acquisition as themeasurement of a simple aggregate directional data signal withoutdifferentiating between the distinct kinds of cardiac electricalactivities presented with an ECG waveform, particularly atrial (P-wave)activity.

The ZIO XT Patch and ZIO Event Card devices, manufactured by iRhythmTech., Inc., San Francisco, Calif., are wearable monitoring devices thatare typically worn on the upper left pectoral region to respectivelyprovide continuous and looping ECG recording. The location is used tosimulate surgically implanted monitors, but without specificallyenhancing P-wave capture. Both of these devices are prescription-onlyand for single patient use. The ZIO XT Patch device is limited to a14-day period, while the electrodes only of the ZIO Event Card devicecan be worn for up to 30 days. The ZIO XT Patch device combines bothelectronic recordation components and physical electrodes into a unitaryassembly that adheres to the patient's skin. The ZIO XT Patch deviceuses adhesive sufficiently strong to support the weight of both themonitor and the electrodes over an extended period and to resistdisadherence from the patient's body, albeit at the cost of disallowingremoval or relocation during the monitoring period. The ZIO Event Carddevice is a form of downsized Holter monitor with a recorder componentthat must be removed temporarily during baths or other activities thatcould damage the non-waterproof electronics. Both devices representcompromises between length of wear and quality of ECG monitoring,especially with respect to ease of long term use, female-friendly fit,and quality of cardiac electrical potential signals, especially atrial(P-wave) signals.

ECG signals contain a large amount of information that requires largestorage space, large transmission bandwidth, and long transmission time.Long-term ECG monitoring further increases the amount of information tobe stored and processed. Data compression is useful in ECG applications,especially long-term monitoring. Data compression can reduce therequirement for data storage space, reduce power consumption, andextends monitoring time. ECG compression can be evaluated based oncompression ratio, signal error loss, and time of execution. A good ECGdata compression preferably should preserve the useful diagnosticinformation while compressing a signal to a smaller acceptable size.

Currently, many Holter monitors use a compression algorithm; however,compression ratios are not satisfactory due to a number of factors. Forexample, the degree of compression achieved may not be sufficient toallow the Holter monitor to conduct a monitoring of a sufficient lengthnecessary for a diagnosis of a cardiac condition while recording thedata of a sufficient quality for diagnosis. Likewise, not allcompression algorithms are suitable for all heart rhythm morphologies.For instance, some compression algorithms are best suited for recordingmonitoring data of a patient with tachyarrhythmia and when applied to apatient with a arterial fibrillation, the compressed data may notpreserve sufficient useful diagnostics necessary for the diagnosis, orthe compressed data may be too large to meet the monitoring durationneed. Further, even if an algorithm is appropriate for compressing ECGmonitoring data of a patient at one point in the monitoring period, thealgorithm may not be suitable at a later point in the monitoring period,when the patient's cardiac activity and rhythm changes.

Therefore, a need remains for a low cost extended wear continuouslyrecording ECG monitor that is capable of self optimizing compressionalgorithm selection to enable efficient monitoring and long termaccuracy.

SUMMARY

Physiological monitoring can be provided through a lightweight wearablemonitor that includes two components, a flexible extended wear electrodepatch and a reusable monitor recorder that removably snaps into areceptacle on the electrode patch. The wearable monitor sits centrally(in the midline) on the patient's chest along the sternum orientedtop-to-bottom. The ECG electrodes on the electrode patch are tailored tobe positioned axially along the midline of the sternum for capturingaction potential propagation in an orientation that corresponds to theaVF lead used in a conventional 12-lead ECG that is used to sensepositive or upright P-waves. The placement of the wearable monitor in alocation at the sternal midline (or immediately to either side of thesternum), with its unique narrow “hourglass”-like shape, significantlyimproves the ability of the wearable monitor to cutaneously sensecardiac electrical potential signals, particularly the P-wave (or atrialactivity) and, to a lesser extent, the QRS interval signals indicatingventricular activity in the ECG waveforms.

Moreover, the electrocardiography monitor offers superior patientcomfort, convenience and user-friendliness. The electrode patch isspecifically designed for ease of use by a patient (or caregiver);assistance by professional medical personnel is not required. Thepatient is free to replace the electrode patch at any time and need notwait for a doctor's appointment to have a new electrode patch placed.Patients can easily be taught to find the familiar physical landmarks onthe body necessary for proper placement of the electrode patch.Empowering patients with the knowledge to place the electrode patch inthe right place ensures that the ECG electrodes will be correctlypositioned on the skin, no matter the number of times that the electrodepatch is replaced. In addition, the monitor recorder operatesautomatically and the patient only need snap the monitor recorder intoplace on the electrode patch to initiate ECG monitoring. Thus, thesynergistic combination of the electrode patch and monitor recordermakes the use of the electrocardiography monitor a reliable andvirtually foolproof way to monitor a patient's ECG and physiology for anextended, or even open-ended, period of time.

Furthermore, the ECG data collected during the long-term monitoring arecompressed through a two-step compression algorithm executed by theelectrocardiography monitor. Minimum amplitude signals may be masked byor become indistinguishable from noise if overly inclusive encoding isemployed in which voltage ranges are set too wide. The resulting ECGsignal will appear “choppy” and uneven with an abrupt (andphysiologically inaccurate and potentially misleading) slope. Thus, theencoding used in the first stage of compression can be dynamicallyrescaled on-the-fly when the granularity of the encoding is too coarse.

Finally, offloaded ECG signals are automatically gained as appropriateon a recording-by-recording basis to preserve the amplitude relationshipbetween the signals. Raw decompressed ECG signals are filtered for noisecontent and any gaps in the signals are bridged. The signal is thengained based on a statistical evaluation of peak-to-peak voltage (orother indicator) to land as many ECG waveforms within a desired range ofdisplay.

In addition, a self-adjusting selection of multiple compressionalgorithms optimized efficient data compression and adaptable to aparticular patient allows the monitor recorder to compress ECGmonitoring data to a degree sufficient to enable long-term monitoringwhile preserving the features important for creating the diagnosis. AsECG waveform characteristics are rarely identical in patients withcardiac disease, the self-optimizing aspect of the compression iscrucial for the long-term data storage and analysis of complex cardiacrhythm disorders. The self-optimizing compression facilitates use ofdevices that are smaller, lighter and more power efficient which iscritical to enabling long term monitoring on patients by improving theircompliance level through enhanced comfort.

In one embodiment, a subcutaneous electrocardiography monitor configuredfor test-based data compression is provided. The monitor includes: animplantable housing for implantation within a patient; a plurality ofelectrocardiographic (ECG) sensing electrodes; electronic circuitryprovided within the housing, the electronic circuitry including: an ECGfront end circuit interfaced to a microcontroller and configured tosense electrocardiographic signals via the ECG sensing electrodes; amemory; and a microcontroller operable to execute under modular microprogram control as specified in firmware. The microcontroller isconfigured to: obtain a series of electrode voltage values based on thesensed electrocardiographic signals; use a plurality of selectionschemes to choose one or more of a plurality of compression algorithmsassociated with each of the selection scheme for testing; test theselected compression algorithms including applying the compressionalgorithms chosen using each of the selection schemes to a segment ofthe electrode voltage series; analyze results of the testing includingcomparing the results of the test achieved using the one or morecompression algorithms using each of the selection schemes; select oneor more compression algorithms chosen using one of the selection schemesfor compressing at least a portion of the electrode voltage series basedon the analysis; obtain a compression of at least the portion of theelectrode voltage series by apply the one or more compression algorithmsselected based on the analysis to at least the portion of the electrodevoltage series; and store the compression within the memory.

In a further embodiment, a cutaneous electrocardiography monitorconfigured for test-based data compression is provided. The monitorincludes a housing configured to fit within a receptacle on a patchapplied to a patient; an electrocardiographic front end circuit withinthe housing that is operable to sense electrocardiographic signalsthrough electrocardiographic electrodes, each electrocardiographicelectrode positioned on one end of the electrode patch: a memory withinthe housing; and a micro-controller within the housing operable toexecute under a micro-programmable control. The micro-controller isconfigured to obtain a series of electrode voltage values based on thesensed electrocardiographic signals; use a plurality of selectionschemes to choose one or more of a plurality of compression algorithmsassociated with each of the selection scheme for testing; test theselected compression algorithms including applying the compressionalgorithms chosen using each of the selection schemes to a segment ofthe electrode voltage series; analyze results of the testing includingcomparing the results of the test achieved using the one or morecompression algorithms using each of the selection schemes; select oneor more compression algorithms chosen using one of the selection schemesfor compressing at least a portion of the electrode voltage series basedon the analysis; obtain a compression of at least the portion of theelectrode voltage series by apply the one or more compression algorithmsselected based on the analysis to at least the portion of the electrodevoltage series; and store the compression within the memory.

The foregoing aspects enhance ECG monitoring performance and quality byfacilitating long-term ECG recording, which is critical to accuratearrhythmia and cardiac rhythm disorder diagnoses. The self optimizingcompression system facilitates use of devices that are smaller, lighterand more power efficient which is critical to enabling long termmonitoring on patients by improving their compliance level throughenhanced comfort.

The monitoring patch is especially suited to the female anatomy,although also easily used over the male sternum. The narrow longitudinalmidsection can fit nicely within the inter-mammary cleft of the breastswithout inducing discomfort, whereas conventional patch electrodes arewide and, if adhered between the breasts, would cause chafing,irritation, discomfort, and annoyance, leading to low patientcompliance.

In addition, the foregoing aspects enhance comfort in women (and certainmen), but not irritation of the breasts, by placing the monitoring patchin the best location possible for optimizing the recording of cardiacsignals from the atrium, particularly P-waves, which is another featurecritical to proper arrhythmia and cardiac rhythm disorder diagnoses.

Still other embodiments will become readily apparent to those skilled inthe art from the following detailed description, wherein are describedembodiments by way of illustrating the best mode contemplated. As willbe realized, other and different embodiments are possible and theembodiments' several details are capable of modifications in variousobvious respects, all without departing from their spirit and the scope.Accordingly, the drawings and detailed description are to be regarded asillustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are diagrams showing, by way of examples, an extended wearelectrocardiography monitor, including an extended wear electrode patch,in accordance with one embodiment, respectively fitted to the sternalregion of a female patient and a male patient.

FIG. 3 is a front anatomical view showing, by way of illustration, thelocations of the heart and lungs within the rib cage of an adult human.

FIG. 4 is a perspective view showing an extended wear electrode patch inaccordance with one embodiment with a monitor recorder inserted.

FIG. 5 is a perspective view showing the monitor recorder of FIG. 4 .

FIG. 6 is a perspective view showing the extended wear electrode patchof FIG. 4 without a monitor recorder inserted.

FIG. 7 is a bottom plan view of the monitor recorder of FIG. 4 .

FIG. 8 is a top view showing the flexible circuit of the extended wearelectrode patch of FIG. 4 .

FIG. 9 is a functional block diagram showing the component architectureof the circuitry of the monitor recorder of FIG. 4 .

FIG. 10 is a functional block diagram showing the circuitry of theextended wear electrode patch of FIG. 4 .

FIG. 11 is a schematic diagram showing the ECG front end circuit of thecircuitry of the monitor recorder of FIG. 9 .

FIG. 12 is a flow diagram showing a monitor recorder-implemented methodfor monitoring ECG data for use in the monitor recorder of FIG. 4 .

FIG. 13 is a graph showing, by way of example, a typical ECG waveform.

FIG. 14 is a functional block diagram showing the signal processingfunctionality of the microcontroller.

FIG. 15 is a functional block diagram showing the operations performedby the download station.

FIGS. 16A-C are functional block diagrams respectively showing practicaluses of the extended wear electrocardiography monitors of FIGS. 1 and 2.

FIG. 17 is a perspective view of an extended wear electrode patch with aflexile wire electrode assembly in accordance with a still furtherembodiment.

FIG. 18 is perspective view of the flexile wire electrode assembly fromFIG. 17 , with a layer of insulating material shielding a bare distalwire around the midsection of the flexible backing.

FIG. 19 is a bottom view of the flexile wire electrode assembly as shownin FIG. 17 .

FIG. 20 is a bottom view of a flexile wire electrode assembly inaccordance with a still yet further embodiment.

FIG. 21 is a perspective view showing the longitudinal midsection of theflexible backing of the electrode assembly from FIG. 17 .

FIG. 22 is a flow diagram showing a monitor recorder-implemented methodfor ECG signal processing and ECG data compressing for use in themonitor recorders of FIG. 4 .

FIG. 23 is a flow diagram showing a monitor recorder-implemented methodfor encoding ECG values.

FIG. 24 is an example of a panel of codes or encodings with each codecovering a range defined by a lower threshold ECG value and an upperthreshold ECG value.

FIG. 25 is an illustrating the encoding and compression scheme inaccordance with method and parameters as described with reference to inFIGS. 23 and 24 .

FIG. 26 is a flow diagram showing a monitor recorder-implemented methodfor further compressing the encodings.

FIG. 27 is a flow diagram showing a routine for providing automatic gaincontrol through middleware.

FIG. 28 is a graph showing, by way of example, an ECG waveform with alow amplitude signal that has been degraded by compression artifacts.

FIG. 29 is a flow diagram showing a routine for providing rescalableencoding.

FIG. 30 is a perspective view showing a unitary extended wear electrodepatch and monitor recorder assembly in accordance with a furtherembodiment.

FIG. 31 is a flow diagram showing a method for selection of an ECG datacompression algorithm in accordance with one embodiment.

FIG. 32 is a flow diagram showing a routine for selecting compressionalgorithms for use in the method of FIG. 31 in accordance with oneembodiment.

FIG. 33 is a flow diagram showing a routine for selecting compressionalgorithms based on characteristics of the waveforms within the seriesfor use in the routine of FIG. 32 .

FIG. 34 is a flow diagram showing a routine for selecting a compressionalgorithm based on empirical testing for use in the routine of FIG. 32in accordance with one embodiment.

FIG. 35 is a routine for applying compression algorithms to a series ofECG values for use in the method of FIG. 31 .

FIG. 36 is a flow diagram showing a routine for an application ofwaveform average compression algorithm for use in the routine of FIG. 35.

FIG. 37 is a flow diagram showing a routine for an application ofwaveform average compression algorithm for use in the routine of FIG. 35in accordance with one embodiment.

FIG. 38 is a flow diagram showing a routine for an application ofwaveform average compression algorithm for use in the routine of FIG. 35in accordance with one embodiment.

FIG. 39 is a flow diagram showing a routine for applying alearning-algorithm-based compression scheme in accordance for use in theroutine of FIG. 35 in accordance with one embodiment.

FIG. 40 is a flow diagram showing a routine for applying adynamic-learning-algorithm-based compression in accordance for use inthe routine of FIG. 35 in accordance with one embodiment.

FIG. 41 is a flow diagram showing a routine for revising the encodingtable for use in the routines of FIGS. 39 and 40 in accordance with oneembodiment.

FIG. 42 is a diagram showing an example of an ICM on which thecompression module can be implemented.

DETAILED DESCRIPTION

ECG and physiological monitoring can be provided through a wearableambulatory monitor that includes two components, a flexible extendedwear electrode patch and a removable reusable (or single use) monitorrecorder. Both the electrode patch and the monitor recorder areoptimized to capture electrical signals from the propagation of lowamplitude, relatively low frequency content cardiac action potentials,particularly the P-waves generated during atrial activation. FIGS. 1 and2 are diagrams showing, by way of examples, an extended wearelectrocardiography monitor 12, including a monitor recorder 14, inaccordance with one embodiment, respectively fitted to the sternalregion of a female patient 10 and a male patient 11. The wearablemonitor 12 sits centrally, positioned axially along the sternal midline16, on the patient's chest along the sternum 13 and orientedtop-to-bottom with the monitor recorder 14 preferably situated towardsthe patient's head. In a further embodiment, the orientation of thewearable monitor 12 can be corrected post-monitoring, as furtherdescribed infra, for instance, if the wearable monitor 12 isinadvertently fitted upside down.

The electrode patch 15 is shaped to fit comfortably and conformal to thecontours of the patient's chest approximately centered on the sternalmidline 16 (or immediately to either side of the sternum 13). The distalend of the electrode patch 15, under which a lower or inferior pole (ECGelectrode) is adhered, extends towards the Xiphoid process and lowersternum and, depending upon the patient's build, may straddle the regionover the Xiphoid process and lower sternum. The proximal end of theelectrode patch 15, located under the monitor recorder 14, under whichan upper or superior pole (ECG electrode) is adhered, is below themanubrium and, depending upon patient's build, may straddle the regionover the manubrium.

During ECG monitoring, the amplitude and strength of action potentialssensed on the body's surface are affected to varying degrees by cardiac,cellular, extracellular, vector of current flow, and physical factors,like obesity, dermatitis, large breasts, and high impedance skin, as canoccur in dark-skinned individuals. Sensing along the sternal midline 16(or immediately to either side of the sternum 13) significantly improvesthe ability of the wearable monitor 12 to cutaneously sense cardiacelectric signals, particularly the P-wave (or atrial activity) and, to alesser extent, the QRS interval signals in the ECG waveforms thatindicate ventricular activity by countering some of the effects of thesefactors.

The ability to sense low amplitude, low frequency content body surfacepotentials is directly related to the location of ECG electrodes on theskin's surface and the ability of the sensing circuitry to capture theseelectrical signals. FIG. 3 is a front anatomical view showing, by way ofillustration, the locations of the heart 4 and lungs 5 within the ribcage of an adult human. Depending upon their placement locations on thechest, ECG electrodes may be separated from activation regions withinthe heart 4 by differing combinations of internal tissues and bodystructures, including heart muscle, intracardiac blood, the pericardium,intrathoracic blood and fluids, the lungs 5, skeletal muscle, bonestructure, subcutaneous fat, and the skin, plus any contaminants presentbetween the skin's surface and electrode signal pickups. The degree ofamplitude degradation of cardiac transmembrane potentials increases withthe number of tissue boundaries between the heart 4 and the skin'ssurface that are encountered. The cardiac electrical field is degradedeach time the transmembrane potentials encounter a physical boundaryseparating adjoining tissues due to differences in the respectivetissues' electrical resistances. In addition, other non-spatial factors,such as pericardial effusion, emphysema or fluid accumulation in thelungs, as further explained infra, can further degrade body surfacepotentials.

Internal tissues and body structures can adversely affect the currentstrength and signal fidelity of all body surface potentials, yet lowamplitude cardiac action potentials, particularly the P-wave with anormative amplitude of less than 0.25 microvolts (mV) and a normativeduration of less than 120 milliseconds (ms), are most apt to benegatively impacted. The atria 6 are generally located posteriorlywithin the thoracic cavity (with the exception of the anterior rightatrium and right atrial appendage), and, physically, the left atriumconstitutes the portion of the heart 4 furthest away from the surface ofthe skin on the chest. Conversely, the ventricles 7, which generatelarger amplitude signals, generally are located anteriorly with theanterior right ventricle and most of the left ventricle situatedrelatively close to the skin surface on the chest, which contributes tothe relatively stronger amplitudes of ventricular waveforms. Thus, thequality of P-waves (and other already-low amplitude action potentialsignals) is more susceptible to weakening from intervening tissues andstructures than the waveforms associated with ventricular activation.

The importance of the positioning of ECG electrodes along the sternalmidline 15 has largely been overlooked by conventional approaches to ECGmonitoring, in part due to the inability of their sensing circuitry toreliably detect low amplitude, low frequency content electrical signals,particularly in P-waves. In turn, that inability to keenly sense P-waveshas motivated ECG electrode placement in other non-sternal midlinethoracic locations, where the QRSTU components that representventricular electrical activity are more readily detectable by theirsensing circuitry than P-waves. In addition, ECG electrode placementalong the sternal midline 15 presents major patient wearabilitychallenges, such as fitting a monitoring ensemble within the narrowconfines of the inter-mammary cleft between the breasts, that to largeextent drive physical packaging concerns, which can be incompatible withECG monitors intended for placement, say, in the upper pectoral regionor other non-sternal midline thoracic locations. In contrast, thewearable monitor 12 uses an electrode patch 15 that is specificallyintended for extended wear placement in a location at the sternalmidline 16 (or immediately to either side of the sternum 13). Whencombined with a monitor recorder 14 that uses sensing circuitryoptimized to preserve the characteristics of low amplitude cardiacaction potentials, especially those signals from the atria, as furtherdescribed infra with reference to FIG. 11 , the electrode patch 15 helpsto significantly improve atrial activation (P-wave) sensing throughplacement in a body location that robustly minimizes the effects oftissue and body structure.

Referring back to FIGS. 1 and 2 , the placement of the wearable monitor12 in the region of the sternal midline 13 puts the ECG electrodes ofthe electrode patch 15 in locations better adapted to sensing andrecording low amplitude cardiac action potentials during atrialpropagation (P-wave signals) than placement in other locations, such asthe upper left pectoral region, as commonly seen in most conventionalambulatory ECG monitors. The sternum 13 overlies the right atrium of theheart 4. As a result, action potential signals have to travel throughfewer layers of tissue and structure to reach the ECG electrodes of theelectrode patch 15 on the body's surface along the sternal midline 13when compared to other monitoring locations, a distinction that is ofcritical importance when capturing low frequency content electricalsignals, such as P-waves.

Moreover, cardiac action potential propagation travels simultaneouslyalong a north-to-south and right-to-left vector, beginning high in theright atrium and ultimately ending in the posterior and lateral regionof the left ventricle. Cardiac depolarization originates high in theright atrium in the SA node before concurrently spreading leftwardtowards the left atrium and inferiorly towards the AV node. The ECGelectrodes of the electrode patch 15 are placed with the upper orsuperior pole (ECG electrode) along the sternal midline 13 in the regionof the manubrium and the lower or inferior pole (ECG electrode) alongthe sternal midline 13 in the region of the Xiphoid process 9 and lowersternum. The ECG electrodes are placed primarily in a north-to-southorientation along the sternum 13 that corresponds to the north-to-southwaveform vector exhibited during atrial activation. This orientationcorresponds to the aVF lead used in a conventional 12-lead ECG that isused to sense positive or upright P-waves.

Furthermore, the thoracic region underlying the sternum 13 along themidline 16 between the manubrium 8 and Xiphoid process 9 is relativelyfree of lung tissue, musculature, and other internal body structuresthat could occlude the electrical signal path between the heart 4,particularly the atria, and ECG electrodes placed on the surface of theskin. Fewer obstructions means that cardiac electrical potentialsencounter fewer boundaries between different tissues. As a result, whencompared to other thoracic ECG sensing locations, the cardiac electricalfield is less altered when sensed dermally along the sternal midline 13.As well, the proximity of the sternal midline 16 to the ventricles 7facilitates sensing of right ventricular activity and provides superiorrecordation of the QRS interval, again, in part due to the relativelyclear electrical path between the heart 4 and the skin surface.

Finally, non-spatial factors can affect transmembrane action potentialshape and conductivity. For instance, myocardial ischemia, an acutecardiac condition, can cause a transient increase in blood perfusion inthe lungs 5. The perfused blood can significantly increase electricalresistance across the lungs 5 and therefore degrade transmission of thecardiac electrical field to the skin's surface. However, the placementof the wearable monitor 12 along the sternal midline 16 in theinter-mammary cleft between the breasts is relatively resilient to theadverse effects to cardiac action potential degradation caused byischemic conditions as the body surface potentials from a locationrelatively clear of underlying lung tissue and fat help compensate forthe loss of signal amplitude and content. The monitor recorder 14 isthus able to record the P-wave morphology that may be compromised bymyocardial ischemia and therefore make diagnosis of the specificarrhythmias that can be associated with myocardial ischemia moredifficult.

During use, the electrode patch 15 is first adhered to the skin alongthe sternal midline 16 (or immediately to either side of the sternum13). A monitor recorder 14 is then snapped into place on the electrodepatch 15 using an electro mechanical docking interface to initiate ECGmonitoring. FIG. 4 is a perspective view showing an extended wearelectrode patch 15 in accordance with one embodiment with a monitorrecorder 14 inserted. The body of the electrode patch 15 is preferablyconstructed using a flexible backing 20 formed as an elongated strip 21of wrap knit or similar stretchable material about 145 mm long and 32 mmat the widest point with a narrow longitudinal mid-section 23 evenlytapering inward from both sides. A pair of cut-outs 22 between thedistal and proximal ends of the electrode patch 15 create a narrowlongitudinal midsection 23 or “isthmus” and defines an elongated“hourglass”-like shape, when viewed from above, such as described incommonly-assigned U.S. Design Pat. No. D744,659, issued Dec. 1, 2015,the disclosure of which is incorporated by reference. The upper part ofthe “hourglass” is sized to allow an electrically non-conductivereceptacle 25, sits on top of the outward-facing surface of theelectrode patch 15, to be affixed to the electrode patch 15 with an ECGelectrode placed underneath on the patient-facing underside, or contact,surface of the electrode patch 15; the upper part of the “hourglass” hasa longer and wider profile (but still rounded and tapered to fitcomfortably between the breasts) than the lower part of the “hourglass,”which is sized primarily to allow just the placement of an ECG electrodeof appropriate shape and surface area to record the P-wave and the QRSsignals sufficiently given the inter-electrode spacing.

The electrode patch 15 incorporates features that significantly improvewearability, performance, and patient comfort throughout an extendedmonitoring period. The entire electrode patch 15 is lightweight inconstruction, which allows the patch to be resilient to disadhesing orfalling off and, critically, to avoid creating distracting discomfort tothe patient, even when the patient is asleep. In contrast, the weight ofa heavy ECG monitor impedes patient mobility and will cause the monitorto constantly tug downwards and press on the patient's body that cangenerate skin inflammation with frequent adjustments by the patientneeded to maintain comfort.

During every day wear, the electrode patch 15 is subjected to pushing,pulling, and torsional movements, including compressional and torsionalforces when the patient bends forward, or tensile and torsional forceswhen the patient leans backwards. To counter these stress forces, theelectrode patch 15 incorporates crimp and strain reliefs, such asdescribed in commonly-assigned U.S. Application Publication No.2015/0087948, the disclosure of which is incorporated by reference. Inaddition, the cut-outs 22 and longitudinal midsection 23 help minimizeinterference with and discomfort to breast tissue, particularly in women(and gynecomastic men). The cut-outs 22 and longitudinal midsection 23further allow better conformity of the electrode patch 15 to sternalbowing and to the narrow isthmus of flat skin that can occur along thebottom of the inter-mammary cleft between the breasts, especially inbuxom women. The cut-outs 22 and narrow and flexible longitudinalmidsection 23 help the electrode patch 15 fit nicely between a pair offemale breasts in the inter-mammary cleft. In one embodiment, thecut-outs 22 can be graduated to form the longitudinal midsection 23 as anarrow in-between stem or isthmus portion about 7 mm wide. In a stillfurther embodiment, tabs 24 can respectively extend an additional 8 mmto 12 mm beyond the distal and proximal ends of the flexible backing 20to facilitate with adhering the electrode patch 15 to or removing theelectrode patch 15 from the sternum 13. These tabs preferably lackadhesive on the underside, or contact, surface of the electrode patch15. Still other shapes, cut-outs and conformities to the electrode patch15 are possible.

The monitor recorder 14 removably and reusably snaps into anelectrically non-conductive receptacle 25 during use. The monitorrecorder 14 contains electronic circuitry for recording and storing thepatient's electrocardiography as sensed via a pair of ECG electrodesprovided on the electrode patch 15, as further described infra beginningwith reference to FIG. 9 . The non-conductive receptacle 25 is providedon the top surface of the flexible backing 20 with a retention catch 26and tension clip 27 molded into the non-conductive receptacle 25 toconformably receive and securely hold the monitor recorder 14 in place.

The electrode patch 15 is generally intended for a single use and ismeant to be replaced periodically throughout an extended period ofmonitoring. However, some types of monitoring may not extend over aperiod of time long enough to necessitate replacement of the electrodepatch 15. In those situations, the monitor recorder 14 and electrodepatch 15 can be combined into a single integral assembly. FIG. 30 is aperspective view showing a unitary extended wear electrode patch andmonitor recorder assembly 300 in accordance with a further embodiment.The monitor recorder 301 and the electrode patch 302 are assembled as asingle unit with the pair of ECG electrodes provided on the electrodepatch 302 electrically connected directly into the circuitry of themonitor recorder 301, thereby obviating the need for a non-conductivereceptacle or other intermediate physical coupling. The assembly 300effectively becomes a single use type of ambulatory monitor with ausable service life dictated by the period of wear. In turn, servicelife can be influenced by the type of adhesive gel used to secure theelectrode patch 302 to the skin and by the patient's tolerance tocontinued wear of the electrode patch 302 on the same spot on the skin.In a still further embodiment, the assembly 300 can be rendered reusableby either employing a form of adhesive gel that permits repeated removaland readherance of the electrode patch 302 or through replenishment ofthe adhesive gel. Still other forms of unitary extended wear electrodepatch and monitor recorder assemblies are possible.

The monitor recorder 14 includes a sealed housing that snaps into placein the non-conductive receptacle 25. FIG. 5 is a perspective viewshowing the monitor recorder 14 of FIG. 4 . The sealed housing 50 of themonitor recorder 14 intentionally has a rounded isoscelestrapezoidal-like shape 52, when viewed from above, such as described incommonly-assigned U.S. Design Pat. No. D717,955, issued Nov. 18, 2014,the disclosure of which is incorporated by reference. The edges 51 alongthe top and bottom surfaces are rounded for patient comfort. The sealedhousing 50 is approximately 47 mm long, 23 mm wide at the widest point,and 7 mm high, excluding a patient-operable tactile-feedback button 55.The sealed housing 50 can be molded out of polycarbonate, ABS, or analloy of those two materials. The button 55 is waterproof and thebutton's top outer surface is molded silicon rubber or similar softpliable material. A retention detent 53 and tension detent 54 are moldedalong the edges of the top surface of the housing 50 to respectivelyengage the retention catch 26 and the tension clip 27 molded intonon-conductive receptacle 25. Other shapes, features, and conformitiesof the sealed housing 50 are possible.

The electrode patch 15 is intended to be disposable, while the monitorrecorder 14 is designed for reuse and can be transferred to successiveelectrode patches 15 to ensure continuity of monitoring, if so desired.The monitor recorder 14 can be used only once, but single useeffectively wastes the synergistic benefits provided by the combinationof the disposable electrode patch and reusable monitor recorder, asfurther explained infra with reference to FIGS. 16A-C. The placement ofthe wearable monitor 12 in a location at the sternal midline 16 (orimmediately to either side of the sternum 13) benefits long-termextended wear by removing the requirement that ECG electrodes becontinually placed in the same spots on the skin throughout themonitoring period. Instead, the patient is free to place an electrodepatch 15 anywhere within the general region of the sternum 13.

As a result, at any point during ECG monitoring, the patient's skin isable to recover from the wearing of an electrode patch 15, whichincreases patient comfort and satisfaction, while the monitor recorder14 ensures ECG monitoring continuity with minimal effort. A monitorrecorder 14 is merely unsnapped from a worn out electrode patch 15, theworn out electrode patch 15 is removed from the skin, a new electrodepatch 15 is adhered to the skin, possibly in a new spot immediatelyadjacent to the earlier location, and the same monitor recorder 14 issnapped into the new electrode patch 15 to reinitiate and continue theECG monitoring.

During use, the electrode patch 15 is first adhered to the skin in thesternal region. FIG. 6 is a perspective view showing the extended wearelectrode patch 15 of FIG. 4 without a monitor recorder 14 inserted. Aflexible circuit 32 is adhered to each end of the flexible backing 20. Adistal circuit trace 33 from the distal end 30 of the flexible backing20 and a proximal circuit trace (not shown) from the proximal end 31 ofthe flexible backing 20 electrically couple ECG electrodes (not shown)with a pair of electrical pads 34. In a further embodiment, the distaland proximal circuit traces are replaced with interlaced or sewn-inflexible wires, as further described infra beginning with reference toFIG. 17 . The electrical pads 34 are provided within amoisture-resistant seal 35 formed on the bottom surface of thenon-conductive receptacle 25. When the monitor recorder 14 is securelyreceived into the non-conductive receptacle 25, that is, snapped intoplace, the electrical pads 34 interface to electrical contacts (notshown) protruding from the bottom surface of the monitor recorder 14.The moisture-resistant seal 35 enables the monitor recorder 14 to beworn at all times, even during showering or other activities that couldexpose the monitor recorder 14 to moisture or adverse conditions.

In addition, a battery compartment 36 is formed on the bottom surface ofthe non-conductive receptacle 25. A pair of battery leads (not shown)from the battery compartment 36 to another pair of the electrical pads34 electrically interface the battery to the monitor recorder 14. Thebattery contained within the battery compartment 35 is a direct current(DC) power cell and can be replaceable, rechargeable or disposable.

The monitor recorder 14 draws power externally from the battery providedin the non-conductive receptacle 25, thereby uniquely obviating the needfor the monitor recorder 14 to carry a dedicated power source. FIG. 7 isa bottom plan view of the monitor recorder 14 of FIG. 4 . A cavity 58 isformed on the bottom surface of the sealed housing 50 to accommodate theupward projection of the battery compartment 36 from the bottom surfaceof the non-conductive receptacle 25, when the monitor recorder 14 issecured in place on the non-conductive receptacle 25. A set ofelectrical contacts 56 protrude from the bottom surface of the sealedhousing 50 and are arranged in alignment with the electrical pads 34provided on the bottom surface of the non-conductive receptacle 25 toestablish electrical connections between the electrode patch 15 and themonitor recorder 14. In addition, a seal coupling 57 circumferentiallysurrounds the set of electrical contacts 56 and securely mates with themoisture-resistant seal 35 formed on the bottom surface of thenon-conductive receptacle 25. The battery contained within the batterycompartment 36 can be replaceable, rechargeable or disposable. In afurther embodiment, the ECG sensing circuitry of the monitor recorder 14can be supplemented with additional sensors, including an SpO₂ sensor, ablood pressure sensor, a temperature sensor, respiratory rate sensor, aglucose sensor, an air flow sensor, and a volumetric pressure sensor,which can be incorporated directly into the monitor recorder 14 or ontothe non-conductive receptacle 25.

The placement of the flexible backing 20 on the sternal midline 16 (orimmediately to either side of the sternum 13) also helps to minimize theside-to-side movement of the wearable monitor 12 in the left- andright-handed directions during wear. However, the wearable monitor 12 isstill susceptible to pushing, pulling, and torqueing movements,including compressional and torsional forces when the patient bendsforward, and tensile and torsional forces when the patient leansbackwards or twists. To counter the dislodgment of the flexible backing20 due to compressional and torsional forces, a layer of non-irritatingadhesive, such as hydrocolloid, is provided at least partially on theunderside, or contact, surface of the flexible backing 20, but only onthe distal end 30 and the proximal end 31. As a result, the underside,or contact surface of the longitudinal midsection 23 does not have anadhesive layer and remains free to move relative to the skin. Thus, thelongitudinal midsection 23 forms a crimp relief that respectivelyfacilitates compression and twisting of the flexible backing 20 inresponse to compressional and torsional forces. Other forms of flexiblebacking crimp reliefs are possible.

Unlike the flexible backing 20, the flexible circuit 32 is only able tobend and cannot stretch in a planar direction. The flexible circuit 32can be provided either above or below the flexible backing 20. FIG. 8 isa top view showing the flexible circuit 32 of the extended wearelectrode patch 15 of FIG. 4 when mounted above the flexible backing 20.A distal ECG electrode 38 and proximal ECG electrode 39 are respectivelycoupled to the distal and proximal ends of the flexible circuit 32 toserve as electrode signal pickups. The flexible circuit 32 preferablydoes not extend to the outside edges of the flexible backing 20, therebyavoiding gouging or discomforting the patient's skin during extendedwear, such as when sleeping on the side. During wear, the ECG electrodes38, 39 must remain in continual contact with the skin. A strain relief40 is defined in the flexible circuit 32 at a location that is partiallyunderneath the battery compartment 36 when the flexible circuit 32 isaffixed to the flexible backing 20. The strain relief 40 is laterallyextendable to counter dislodgment of the ECG electrodes 38, 39 due tobending, tensile and torsional forces. A pair of strain relief cutouts41 partially extend transversely from each opposite side of the flexiblecircuit 32 and continue longitudinally towards each other to define in‘S’-shaped pattern, when viewed from above. The strain reliefrespectively facilitates longitudinal extension and twisting of theflexible circuit 32 in response to tensile and torsional forces. Otherforms of circuit board strain relief are possible.

ECG monitoring and other functions performed by the monitor recorder 14are provided through a micro controlled architecture. FIG. 9 is afunctional block diagram showing the component architecture of thecircuitry 60 of the monitor recorder 14 of FIG. 4 . The circuitry 60 isexternally powered through a battery provided in the non-conductivereceptacle 25 (shown in FIG. 6 ). Both power and raw ECG signals, whichoriginate in the pair of ECG electrodes 38, 39 (shown in FIG. 8 ) on thedistal and proximal ends of the electrode patch 15, are received throughan external connector 65 that mates with a corresponding physicalconnector on the electrode patch 15. The external connector 65 includesthe set of electrical contacts 56 that protrude from the bottom surfaceof the sealed housing 50 and which physically and electrically interfacewith the set of pads 34 provided on the bottom surface of thenon-conductive receptacle 25. The external connector includes electricalcontacts 56 for data download, microcontroller communications, power,analog inputs, and a peripheral expansion port. The arrangement of thepins on the electrical connector 65 of the monitor recorder 14 and thedevice into which the monitor recorder 14 is attached, whether anelectrode patch 15 or download station (not shown), follow the sameelectrical pin assignment convention to facilitate interoperability. Theexternal connector 65 also serves as a physical interface to a downloadstation that permits the retrieval of stored ECG monitoring data,communication with the monitor recorder 14, and performance of otherfunctions. The download station is further described infra withreference to FIG. 15 .

Operation of the circuitry 60 of the monitor recorder 14 is managed by amicrocontroller 61, such as the EFM32 Tiny Gecko 32-bit microcontroller,manufactured by Silicon Laboratories Inc., Austin, Tex. Themicrocontroller 61 has flexible energy management modes and includes adirect memory access controller and built-in analog-to-digital anddigital-to-analog converters (ADC and DAC, respectively). Themicrocontroller 61 also includes a program memory unit containinginternal flash memory that is readable and writeable. The internal flashmemory can also be programmed externally. The microcontroller 61operates under modular micro program control as specified in firmwarestored in the internal flash memory. The functionality and firmwaremodules relating to signal processing by the microcontroller 61 arefurther described infra with reference to FIG. 14 . The microcontroller61 draws power externally from the battery provided on the electrodepatch 15 via a pair of the electrical contacts 56. The microcontroller61 connects to the ECG front end circuit 63 that measures raw cutaneouselectrical signals using a driven reference that eliminates common modenoise, as further described infra with reference to FIG. 11 .

The circuitry 60 of the monitor recorder 14 also includes a flash memory62, which the microcontroller 61 uses for storing ECG monitoring dataand other physiology and information. The flash memory 62 also drawspower externally from the battery provided on the electrode patch 15 viaa pair of the electrical contacts 56. Data is stored in a serial flashmemory circuit, which supports read, erase and program operations over acommunications bus. The flash memory 62 enables the microcontroller 61to store digitized ECG data. The communications bus further enables theflash memory 62 to be directly accessed externally over the externalconnector 65 when the monitor recorder 14 is interfaced to a downloadstation. In a further embodiment, the memory 62 can be a volatilememory.

The microcontroller 61 includes functionality that enables theacquisition of samples of analog ECG signals, which are converted into adigital representation, as further described infra with reference toFIG. 14 . In one mode, the microcontroller 61 will acquire, sample,digitize, signal process, and store digitized ECG data into availablestorage locations in the flash memory 62 until all memory storagelocations are filled, after which the digitized ECG data needs to bedownloaded or erased to restore memory capacity. Data download orerasure can also occur before all storage locations are filled, whichwould free up memory space sooner, albeit at the cost of possiblyinterrupting monitoring while downloading or erasure is performed. Inanother mode, the microcontroller 61 can include a loop recorder featurethat will overwrite the oldest stored data once all storage locationsare filled, albeit at the cost of potentially losing the stored datathat was overwritten, if not previously downloaded. Still other modes ofdata storage and capacity recovery are possible.

The circuitry 60 of the monitor recorder 14 further includes anactigraphy sensor 64 implemented as a 3-axis accelerometer. Theaccelerometer may be configured to generate interrupt signals to themicrocontroller 61 by independent initial wake up and free fall events,as well as by device position. In addition, the actigraphy provided bythe accelerometer can be used during post-monitoring analysis to correctthe orientation of the monitor recorder 14 if, for instance, the monitorrecorder 14 has been inadvertently installed upside down, that is, withthe monitor recorder 14 oriented on the electrode patch 15 towards thepatient's feet, as well as for other event occurrence analyses.

The microcontroller 61 includes an expansion port that also utilizes thecommunications bus. External devices, separately drawing powerexternally from the battery provided on the electrode patch 15 or othersource, can interface to the microcontroller 61 over the expansion portin half duplex mode. For instance, an external physiology sensor can beprovided as part of the circuitry 60 of the monitor recorder 14, or canbe provided on the electrode patch 15 with communication with themicrocontroller 61 provided over one of the electrical contacts 56. Thephysiology sensor can include an SpO₂ sensor, blood pressure sensor,temperature sensor, respiratory rate sensor, glucose sensor, airflowsensor, volumetric pressure sensing, or other types of sensor ortelemetric input sources. In a further embodiment, a wireless interfacefor interfacing with other wearable (or implantable) physiologymonitors, as well as data offload and programming, can be provided aspart of the circuitry 60 of the monitor recorder 14, or can be providedon the electrode patch 15 with communication with the microcontroller 61provided over one of the electrical contacts 56.

Finally, the circuitry 60 of the monitor recorder 14 includespatient-interfaceable components, including a tactile feedback button66, which a patient can press to mark events or to perform otherfunctions, and a buzzer 67, such as a speaker, magnetic resonator orpiezoelectric buzzer. The buzzer 67 can be used by the microcontroller61 to output feedback to a patient such as to confirm power up andinitiation of ECG monitoring. Still other components as part of thecircuitry 60 of the monitor recorder 14 are possible.

While the monitor recorder 14 operates under micro control, most of theelectrical components of the electrode patch 15 operate passively. FIG.10 is a functional block diagram showing the circuitry 70 of theextended wear electrode patch 15 of FIG. 4 . The circuitry 70 of theelectrode patch 15 is electrically coupled with the circuitry 60 of themonitor recorder 14 through an external connector 74. The externalconnector 74 is terminated through the set of pads 34 provided on thebottom of the non-conductive receptacle 25, which electrically mate tocorresponding electrical contacts 56 protruding from the bottom surfaceof the sealed housing 50 to electrically interface the monitor recorder14 to the electrode patch 15.

The circuitry 70 of the electrode patch 15 performs three primaryfunctions. First, a battery 71 is provided in a battery compartmentformed on the bottom surface of the non-conductive receptacle 25. Thebattery 71 is electrically interfaced to the circuitry 60 of the monitorrecorder 14 as a source of external power. The unique provisioning ofthe battery 71 on the electrode patch 15 provides several advantages.First, the locating of the battery 71 physically on the electrode patch15 lowers the center of gravity of the overall wearable monitor 12 andthereby helps to minimize shear forces and the effects of movements ofthe patient and clothing. Moreover, the housing 50 of the monitorrecorder 14 is sealed against moisture and providing power externallyavoids having to either periodically open the housing 50 for the batteryreplacement, which also creates the potential for moisture intrusion andhuman error, or to recharge the battery, which can potentially take themonitor recorder 14 off line for hours at a time. In addition, theelectrode patch 15 is intended to be disposable, while the monitorrecorder 14 is a reusable component. Each time that the electrode patch15 is replaced, a fresh battery is provided for the use of the monitorrecorder 14, which enhances ECG monitoring performance quality andduration of use. Also, the architecture of the monitor recorder 14 isopen, in that other physiology sensors or components can be added byvirtue of the expansion port of the microcontroller 61. Requiring thoseadditional sensors or components to draw power from a source external tothe monitor recorder 14 keeps power considerations independent of themonitor recorder 14. This approach also enables a battery of highercapacity to be introduced when needed to support the additional sensorsor components without effecting the monitor recorders circuitry 60.

Second, the pair of ECG electrodes 38, 39 respectively provided on thedistal and proximal ends of the flexible circuit 32 are electricallycoupled to the set of pads 34 provided on the bottom of thenon-conductive receptacle 25 by way of their respective circuit traces33, 37. The signal ECG electrode 39 includes a protection circuit 72,which is an inline resistor that protects the patient from excessiveleakage current should the front end circuit fail.

Last, in a further embodiment, the circuitry 70 of the electrode patch15 includes a cryptographic circuit 73 to authenticate an electrodepatch 15 for use with a monitor recorder 14. The cryptographic circuit73 includes a device capable of secure authentication and validation.The cryptographic device 73 ensures that only genuine, non-expired,safe, and authenticated electrode patches 15 are permitted to providemonitoring data to a monitor recorder 14 and for a specific patient.

The ECG front end circuit 63 measures raw cutaneous electrical signalsusing a driven reference that effectively reduces common mode noise,power supply noise and system noise, which is critical to preserving thecharacteristics of low amplitude cardiac action potentials, especiallythose signals from the atria. FIG. 11 is a schematic diagram 80 showingthe ECG front end circuit 63 of the circuitry 60 of the monitor recorder14 of FIG. 9 . The ECG front end circuit 63 senses body surfacepotentials through a signal lead (“S1”) and reference lead (“REF”) thatare respectively connected to the ECG electrodes of the electrode patch15. Power is provided to the ECG front end circuit 63 through a pair ofDC power leads (“VCC” and “GND”). An analog ECG signal (“ECG”)representative of the electrical activity of the patient's heart overtime is output, which the micro controller 11 converts to digitalrepresentation and filters, as further described infra.

The ECG front end circuit 63 is organized into five stages, a passiveinput filter stage 81, a unity gain voltage follower stage 82, a passivehigh pass filtering stage 83, a voltage amplification and activefiltering stage 84, and an anti-aliasing passive filter stage 85, plus areference generator. Each of these stages and the reference generatorwill now be described.

The passive input filter stage 81 includes the parasitic impedance ofthe ECG electrodes 38, 39 (shown in FIG. 8 ), the protection resistorthat is included as part of the protection circuit 72 of the ECGelectrode 39 (shown in FIG. 10 ), an AC coupling capacitor 87, atermination resistor 88, and filter capacitor 89. This stage passivelyshifts the frequency response poles downward there is a high electrodeimpedance from the patient on the signal lead S1 and reference lead REF,which reduces high frequency noise.

The unity gain voltage follower stage 82 provides a unity voltage gainthat allows current amplification by an Operational Amplifier (“Op Amp”)90. In this stage, the voltage stays the same as the input, but morecurrent is available to feed additional stages. This configurationallows a very high input impedance, so as not to disrupt the bodysurface potentials or the filtering effect of the previous stage.

The passive high pass filtering stage 83 is a high pass filter thatremoves baseline wander and any offset generated from the previousstage. Adding an AC coupling capacitor 91 after the Op Amp 90 allows theuse of lower cost components, while increasing signal fidelity.

The voltage amplification and active filtering stage 84 amplifies thevoltage of the input signal through Op Amp 91, while applying a low passfilter. The DC bias of the input signal is automatically centered in thehighest performance input region of the Op Amp 91 because of the ACcoupling capacitor 91.

The anti-aliasing passive filter stage 85 provides an anti-aliasing lowpass filter. When the microcontroller 61 acquires a sample of the analoginput signal, a disruption in the signal occurs as a sample and holdcapacitor that is internal to the microcontroller 61 is charged tosupply signal for acquisition.

The reference generator in subcircuit 86 drives a driven referencecontaining power supply noise and system noise to the reference leadREF. A coupling capacitor 87 is included on the signal lead S1 and apair of resistors 93 a, 93 b inject system noise into the reference leadREF. The reference generator is connected directly to the patient,thereby avoiding the thermal noise of the protection resistor that isincluded as part of the protection circuit 72.

In contrast, conventional ECG lead configurations try to balance signaland reference lead connections. The conventional approach suffers fromthe introduction of differential thermal noise, lower input common moderejection, increased power supply noise, increased system noise, anddifferential voltages between the patient reference and the referenceused on the device that can obscure, at times, extremely, low amplitudebody surface potentials.

Here, the parasitic impedance of the ECG electrodes 38, 39, theprotection resistor that is included as part of the protection circuit72 and the coupling capacitor 87 allow the reference lead REF to beconnected directly to the skin's surface without any further components.As a result, the differential thermal noise problem caused by pairingprotection resistors to signal and reference leads, as used inconventional approaches, is avoided.

The monitor recorder 14 continuously monitors the patient's heart rateand physiology. FIG. 12 is a flow diagram showing a monitorrecorder-implemented method 100 for monitoring ECG data for use in themonitor recorder 14 of FIG. 4 . Initially, upon being connected to theset of pads 34 provided with the non-conductive receptacle 25 when themonitor recorder 14 is snapped into place, the microcontroller 61executes a power up sequence (step 101). During the power up sequence,the voltage of the battery 71 is checked, the state of the memory 62 isconfirmed, both in terms of operability check and available capacity,and microcontroller operation is diagnostically confirmed. In a furtherembodiment, an authentication procedure between the microcontroller 61and the electrode patch 15 are also performed.

Following satisfactory completion of the power up sequence, an iterativeprocessing loop (steps 102-110) is continually executed by themicrocontroller 61. During each iteration (step 102) of the processingloop, the ECG frontend 63 (shown in FIG. 9 ) continually senses thecutaneous ECG electrical signals (step 103) via the ECG electrodes 38,29 and is optimized to maintain the integrity of the P-wave. A sample ofthe ECG signal is read (step 104) by the microcontroller 61 by samplingthe analog ECG signal that is output by the ECG front end circuit 63.FIG. 13 is a graph showing, by way of example, a typical ECG waveform120. The x-axis represents time in approximate units of tenths of asecond. The y-axis represents cutaneous electrical signal strength inapproximate units of millivolts. The P-wave 121 has a smooth, normallyupward, that is, positive, waveform that indicates atrialdepolarization. The QRS complex often begins with the downwarddeflection of a Q-wave 122, followed by a larger upward deflection of anR-wave 123, and terminated with a downward waveform of the S-wave 124,collectively representative of ventricular depolarization. The T-wave125 is normally a modest upward waveform, representative of ventriculardepolarization, while the U-wave 126, often not directly observable,indicates the recovery period of the Purkinje conduction fibers.

Sampling of the R-to-R interval enables heart rate informationderivation. For instance, the R-to-R interval represents the ventricularrate and rhythm, while the P-to-P interval represents the atrial rateand rhythm. Importantly, the PR interval is indicative ofatrioventricular (AV) conduction time and abnormalities in the PRinterval can reveal underlying heart disorders, thus representinganother reason why the P-wave quality achievable by the ambulatoryelectrocardiography monitoring patch optimized for capturing lowamplitude cardiac action potential propagation described herein ismedically unique and important. The long-term observation of these ECGindicia, as provided through extended wear of the wearable monitor 12,provides valuable insights to the patient's cardiac function symptoms,and overall well-being.

Referring back to FIG. 12 , each sampled ECG signal, in quantized anddigitized form, is processed by signal processing modules as specifiedin firmware (step 105), as described infra, and temporarily staged in abuffer (step 106), pending compression preparatory to storage in thememory 62 (step 107). Following compression, the compressed ECGdigitized sample is again buffered (step 108), then written to thememory 62 (step 109) using the communications bus. Processing continues(step 110), so long as the monitoring recorder 14 remains connected tothe electrode patch 15 (and storage space remains available in thememory 62), after which the processing loop is exited (step 110) andexecution terminates. Still other operations and steps are possible.

The microcontroller 61 operates under modular micro program control asspecified in firmware, and the program control includes processing ofthe analog ECG signal output by the ECG front end circuit 63. FIG. 14 isa functional block diagram showing the signal processing functionality130 of the microcontroller 61. The microcontroller 61 operates undermodular micro program control as specified in firmware 132. The firmwaremodules 132 include high and low pass filtering 133, and compression134. Other modules are possible. The microcontroller 61 has a built-inADC, although ADC functionality could also be provided in the firmware132.

The ECG front end circuit 63 first outputs an analog ECG signal, whichthe ADC 131 acquires, samples and converts into an uncompressed digitalrepresentation. The microcontroller 61 includes one or more firmwaremodules 133 that perform filtering. In one embodiment, three low passfilters and two high pass filters are used. Following filtering, thedigital representation of the cardiac activation wave front amplitudesare compressed by a compression module 134 before being written out tostorage 135, as further described infra beginning with reference to FIG.22 .

The download station executes a communications or offload program(“Offload”) or similar program that interacts with the monitor recorder14 via the external connector 65 to retrieve the stored ECG monitoringdata. FIG. 15 is a functional block diagram showing the operations 140performed by the download station. The download station could be aserver, personal computer, tablet or handheld computer, smart mobiledevice, or purpose-built programmer designed specific to the task ofinterfacing with a monitor recorder 14. Still other forms of downloadstation are possible, including download stations connected throughwireless interfacing using, for instance, a smart phone connected to themonitor recorder 14 through Bluetooth or Wi-Fi.

The download station is responsible for offloading stored ECG monitoringdata from a monitor recorder 14 and includes an electro mechanicaldocking interface by which the monitor recorder 14 is connected at theexternal connector 65. The download station operates under programmablecontrol as specified in software 141. The stored ECG monitoring dataretrieved from storage 142 on a monitor recorder 14 is firstdecompressed by a decompression module 143, which decompresses thestored ECG monitoring data back into an uncompressed digital time seriesrepresentation of the raw ECG signal that is more suited to signalprocessing than a compressed signal. The retrieved ECG monitoring datamay be stored into local storage for archival purposes, either inoriginal compressed form, or as an uncompressed time series.

The download station can include an array of filtering modules. Forinstance, a set of phase distortion filtering tools 144 may be provided,where corresponding software filters can be provided for each filterimplemented in the firmware executed by the microcontroller 61. Thedigital signals are run through the software filters in a reversedirection to remove phase distortion. For instance, a 45 Hertz high passfilter in firmware may have a matching reverse 45 Hertz high pass filterin software. Most of the phase distortion is corrected, that is,canceled to eliminate noise at the set frequency, but data at otherfrequencies in the waveform remain unaltered. As well, bidirectionalimpulse infinite response (IIR) high pass filters and reverse direction(symmetric) IIR low pass filters can be provided. Data is run throughthese filters first in a forward direction, then in a reverse direction,which generates a square of the response and cancels out any phasedistortion. This type of signal processing is particularly helpful withimproving the display of the ST-segment by removing low frequency noise.

The download station can also include filtering modules specificallyintended to enhance P-wave content. For instance, a P-wave base boostfilter 145, which is a form of pre-emphasis filter, can be applied tothe signal to restore missing frequency content or to correct phasedistortion. Still other filters and types of signal processing arepossible.

An automatic gain control (AGC) module 147 can be provided as part of asuite of post-processing modules 146 that include ECG signalvisualization 148. The AGC module 147 adjusts the digital signals to ausable level based on peak or average signal level or other metric. AGCis particularly critical to single-lead ECG monitors, where physicalfactors, such as the tilt of the heart or region of placement on thebody, can affect the electrical field generated. On three-lead Holtermonitors, the leads are oriented in vertical, horizontal and diagonaldirections. As a result, the horizontal and diagonal leads may be higheramplitude and ECG interpretation will be based on one or both of thehigher amplitude leads. In contrast, the electrocardiography monitor 12has only a single lead that is oriented in the vertical direction, sovariations in amplitude will be wider than available with multi-leadmonitors, which have alternate leads to fall back upon.

In addition, AGC may be necessary to maintain compatibility withexisting ECG interpretation software, which is typically calibrated formulti-lead ECG monitors for viewing signals over a narrow range ofamplitudes. Through the AGC module 147, the gain of signals recorded bythe monitor recorder 14 of the electrocardiography monitor 12 can beattenuated up (or down) to work with FDA-approved commercially availableECG interpretation.

AGC can be implemented in a fixed fashion that is uniformly applied toall signals in an ECG recording, adjusted as appropriate on arecording-by-recording basis. Typically, a fixed AGC value is calculatedbased on how an ECG recording is received to preserve the amplituderelationship between the signals. Alternatively, AGC can be varieddynamically throughout an ECG recording, where signals in differentsegments of an ECG recording are amplified up (or down) by differingamounts of gain or to maximize the number of signals appearing within adesired range.

Typically, the monitor recorder 14 will record a high resolution, lowfrequency signal for the P-wave segment. However, for some patients, theresult may still be a visually small signal that can complicatediagnostic overread. Although high signal resolution is present, theunaided eye will normally be unable to discern the P-wave segment withsufficient detail. Therefore, gaining the signal can be critical tovisually depicting P-wave details with proper clarity, which can beautomatically performed by middleware after a set of ECG signals hasbeen downloaded from a monitor recorder 14 and decompressed. FIG. 27 isa flow diagram showing a routine 260 for providing automatic gaincontrol through middleware. Automatic gain control is generally providedthrough an AGC module 147 that is implemented as part of a suite ofpost-processing modules 146, although automatic gain control could alsobe provided through standalone software or as part of other forms of ECGvisualization and interpretation software.

The AGC technique works most efficaciously with a raw (decompressed) ECGsignal with low noise and high resolution. Noise amplitude differs fromthe signal amplitude, so noise must be excluded from the gaincalculations. Automatic gain control applied to a high noise signal willonly exacerbate noise content and be self-defeating. Thus, the routine260 first applies a set of filters to identify noise in the signal(steps 263-268). The noise is then marked and subsequently ignored,except as noted, during automatic signal gain (steps 272-275). Duringnoise filtering, a set of noise filter parameters is first defined (step261). The noise filter parameters are dependent upon the type of filteremployed, as set forth infra. The set of raw decompressed ECG values areobtained (step 262).

Each ECG value is processed (step 263) in an iterative loop (steps263-268). For each ECG value (step 263), four types of noise filters areapplied to the ECG values, a high pass filter (step 264), a low passfilter (step 265), an amplitude change filter (step 266), and azero-crossings filter (step 267). As used herein, “peak-to-peak” meansthe difference between the maximum and minimum voltage during someparticular time period. Ordinarily, peak ECG voltage occurs at the topof the upward deflection of the R-wave 123 (shown in FIG. 13 ), althoughother peak ECG voltage markers could be used, such as the top of theupward deflection of consecutive P-waves 121, so long as the same twotypes of peak ECG voltages are compared. The high pass filter (step 264)marks signals that fall below as pre-defined threshold as noise. Thefilter is triggered when eight seconds or more of sub-0.2 mV signals,peak-to-peak, are detected, and a span of about 120 seconds of thesignal is marked as noise, starting from the eight-second point. Otherhigh pass threshold values and filtration durations could be employed.Note that the noise markers are ignored for purposes of automatic signalgain. The low pass filter (step 265) marks signals that fall above apre-defined threshold as noise. The filter is triggered wheneversupra-20 mV signals, peak-to-peak, are detected, and a span of about 60seconds is marked as noise, 30 seconds preceding and 30 secondsfollowing the first detection of noise. Other threshold values andfiltration durations could be employed. The amplitude change filter(step 266) marks signals whose difference from immediately precedingsignals exceed a pre-defined threshold as noise. The filter is triggeredwhen the peak-to-peak signal strengths of adjoining three-secondsegments change by more than 300%, that is, triples in strength, and aspan of about 30 seconds is marked as noise, starting from the onset ofnoise. Other threshold values and filtration durations could beemployed. Finally, the zero-crossings filter (step 267) marks signalsthat cross zero mV, from either direction, more than a pre-definedthreshold as noise. The filter is triggered when the signal crosses zeromore than ten times in one second, and a span of about 15 seconds ismarked as noise, starting with the onset of noise. Other zero-crossingsthreshold values and filtration durations could be employed. Other typesof noise filters are also possible. Each remaining ECG value isprocessed (step 268).

Automatic gain is applied to enhance an ECG signal to help displayimportant details, especially visually depicting P-wave details.Automatic gain is applied to the ECG signals after noise has been marked(steps 263-268), although noise filtering, while recommended anddesirable, could be skipped. Finally, gaps of non-noise are bridged(step 269), so that the displayed ECG signal appears to be a continuoussignal. The appearance of continuity of signal can be particularlyimportant when long gaps of noise are present. Note that noise in thesignal is only marked. The raw, unfiltered signal, complete with noise,can still be retrieved and displayed, if desired.

Initially, a temporal window for AGC is defined (step 270). The temporalwindow has to be long enough to capture at least one heart beat; afive-second temporal window is used, although other durations arepossible. The peak-to-peak voltage of each noiseless window is computed(step 271). As the goal is to try to place or center as manypeak-to-peak voltages in the preferred range as practicable, a singlegain factor is applied to the entire signal such that the average signalfalls in the center of the preferred range. Alternatively, otherstatistical values could be used to represent the correlation of thepeak-to-peak voltages in the preferred range, either in lieu of or inaddition to the average voltage, such as maximum or minimum observedvoltage, mean voltage, and so on.

Each ECG value is then processed (step 273) in an iterative loop (steps273-275). The ECG values are dynamically gained (step 274). In oneembodiment, the preferred range falls from about 2 mV to 10 mV, althoughother values could be chosen. Each remaining ECG value is processed(step 275).

Conventional ECG monitors, like Holter monitors, invariably requirespecialized training on proper placement of leads and on the operationof recording apparatuses, plus support equipment purpose-built toretrieve, convert, and store ECG monitoring data. In contrast, theelectrocardiography monitor 12 simplifies monitoring from end to end,starting with placement, then with use, and finally with data retrieval.FIGS. 16A-C are functional block diagrams respectively showing practicaluses 150, 160, 170 of the extended wear electrocardiography monitors 12of FIGS. 1 and 2 . The combination of a flexible extended wear electrodepatch and a removable reusable (or single use) monitor recorder empowersphysicians and patients alike with the ability to readily performlong-term ambulatory monitoring of the ECG and physiology.

Especially when compared to existing Holter-type monitors and monitoringpatches placed in the upper pectoral region, the electrocardiographymonitor 12 offers superior patient comfort, convenience anduser-friendliness. To start, the electrode patch 15 is specificallydesigned for ease of use by a patient (or caregiver); assistance byprofessional medical personnel is not required. Moreover, the patient isfree to replace the electrode patch 15 at any time and need not wait fora doctor's appointment to have a new electrode patch 15 placed. Inaddition, the monitor recorder 14 operates automatically and the patientonly need snap the monitor recorder 14 into place on the electrode patch15 to initiate ECG monitoring. Thus, the synergistic combination of theelectrode patch 15 and monitor recorder 14 makes the use of theelectrocardiography monitor 12 a reliable and virtually foolproof way tomonitor a patient's ECG and physiology for an extended, or evenopen-ended, period of time.

In simplest form, extended wear monitoring can be performed by using thesame monitor recorder 14 inserted into a succession of fresh newelectrode patches 15. As needed, the electrode patch 15 can be replacedby the patient (or caregiver) with a fresh new electrode patch 15throughout the overall monitoring period. Referring first to FIG. 16A,at the outset of monitoring, a patient adheres a new electrode patch 15in a location at the sternal midline 16 (or immediately to either sideof the sternum 13) oriented top-to-bottom (step 151). The placement ofthe wearable monitor in a location at the sternal midline (orimmediately to either side of the sternum), with its unique narrow“hourglass”-like shape, significantly improves the ability of thewearable monitor to cutaneously sense cardiac electrical potentialsignals, particularly the P-wave (or atrial activity) and, to a lesserextent, the QRS interval signals indicating ventricular activity in theECG waveforms.

Placement involves simply adhering the electrode patch 15 on the skinalong the sternal midline 16 (or immediately to either side of thesternum 13). Patients can easily be taught to find the physicallandmarks on the body necessary for proper placement of the electrodepatch 15. The physical landmarks are locations on the surface of thebody that are already familiar to patients, including the inter-mammarycleft between the breasts above the manubrium (particularly easilylocatable by women and gynecomastic men), the sternal notch immediatelyabove the manubrium, and the Xiphoid process located at the bottom ofthe sternum. Empowering patients with the knowledge to place theelectrode patch 15 in the right place ensures that the ECG electrodeswill be correctly positioned on the skin, no matter the number of timesthat the electrode patch 15 is replaced.

A monitor recorder 14 is snapped into the non-conductive receptacle 25on the outward-facing surface of the electrode patch 15 (step 152). Themonitor recorder 14 draws power externally from a battery provided inthe non-conductive receptacle 25. In addition, the battery is replacedeach time that a fresh new electrode patch 15 is placed on the skin,which ensures that the monitor recorder 14 is always operating with afresh power supply and minimizing the chances of a loss of monitoringcontinuity due to a depleted battery source.

By default, the monitor recorder 14 automatically initiates monitoringupon sensing body surface potentials through the pair of ECG electrodes(step 153). In a further embodiment, the monitor recorder 14 can beconfigured for manual operation, such as by using the tactile feedbackbutton 66 on the outside of the sealed housing 50, or otheruser-operable control. In an even further embodiment, the monitorrecorder 14 can be configured for remotely-controlled operation byequipping the monitor recorder 14 with a wireless transceiver, such asdescribed in commonly-assigned U.S. Pat. No. 9,433,367, issued Sep. 6,2016, the disclosure of which is incorporated by reference. The wirelesstransceiver allows wearable or mobile communications devices towirelessly interface with the monitor recorder 14.

A key feature of the extended wear electrocardiography monitor 12 is theability to monitor ECG and physiological data for an extended period oftime, which can be well in excess of the 14 days currently pitched asbeing achievable by conventional ECG monitoring approaches. In a furtherembodiment, ECG monitoring can even be performed over an open-ended timeperiod, as further explained infra. The monitor recorder 14 is reusableand, if so desired, can be transferred to successive electrode patches15 to ensure continuity of monitoring. At any point during ECGmonitoring, a patient (or caregiver) can remove the monitor recorder 14(step 154) and replace the electrode patch 15 currently being worn witha fresh new electrode patch 15 (step 151). The electrode patch 15 mayneed to be replaced for any number of reasons. For instance, theelectrode patch 15 may be starting to come off after a period of wear orthe patient may have skin that is susceptible to itching or irritation.The wearing of ECG electrodes can aggravate such skin conditions. Thus,a patient may want or need to periodically remove or replace ECGelectrodes during a long-term ECG monitoring period, whether to replacea dislodged electrode, reestablish better adhesion, alleviate itching orirritation, allow for cleansing of the skin, allow for showering andexercise, or for other purpose.

Following replacement, the monitor recorder 14 is again snapped into theelectrode patch 15 (step 152) and monitoring resumes (step 153). Theability to transfer the same monitor recorder 14 to successive electrodepatches 15 during a period of extended wear monitoring is advantageousnot to just diagnose cardiac rhythm disorders and other physiologicalevents of potential concern, but to do extremely long term monitoring,such as following up on cardiac surgery, ablation procedures, or medicaldevice implantation. In these cases, several weeks of monitoring or moremay be needed. In addition, some IMDs, such as pacemakers or implantablecardioverter defibrillators, incorporate a loop recorder that willcapture cardiac events over a fixed time window. If the telemetryrecorded by the IMD is not downloaded in time, cardiac events thatoccurred at a time preceding the fixed time window will be overwrittenby the IMD and therefore lost. The monitor recorder 14 providescontinuity of monitoring that acts to prevent loss of cardiac eventdata. In a further embodiment, the firmware executed by themicrocontroller 61 of the monitor recorder 14 can be optimized forminimal power consumption and additional memory for storing monitoringdata can be added to achieve a multi-week monitor recorder 14 that canbe snapped into a fresh new electrode patch 15 every seven days, orother interval, for weeks or even months on end.

Upon the conclusion of monitoring, the monitor recorder 14 is removed(step 154) and recorded ECG and physiological telemetry are downloaded(step 155). For instance, a download station can be physicallyinterfaced to the external connector 65 of the monitor recorder 14 toinitiate and conduct downloading, as described supra with reference toFIG. 15 .

In a further embodiment, the monitoring period can be of indeterminateduration. Referring next to FIG. 16B, a similar series of operations arefollowed with respect to replacement of electrode patches 15,reinsertion of the same monitor recorder 14, and eventual download ofECG and physiological telemetry (steps 161-165), as described supra withreference to FIG. 16A. However, the memory 62 (shown in FIG. 9 ) in thecircuitry 60 of the monitor recorder 14 has a finite capacity. Followingsuccessful downloading of stored data, the memory 62 can be cleared torestore storage capacity and monitoring can resume once more, either byfirst adhering a new electrode patch 15 (step 161) or by snapping themonitor recorder 14 into an already-adhered electrode patch 15 (step162). The foregoing expanded series of operations, to include reuse ofthe same monitor recorder 14 following data download, allows monitoringto continue indefinitely and without the kinds of interruptions thatoften affect conventional approaches, including the retrieval ofmonitoring data only by first making an appointment with a medicalprofessional.

In a still further embodiment, when the monitor recorder 14 is equippedwith a wireless transceiver, the use of a download station can beskipped. Referring last to FIG. 16C, a similar series of operations arefollowed with respect to replacement of electrode patches 15 andreinsertion of the same monitor recorder 14 (steps 171-174), asdescribed supra with reference to FIG. 16A. However, recorded ECG andphysiological telemetry are downloaded wirelessly (step 175), such asdescribed in commonly-assigned U.S. patent application Ser. No.14/082,071, cited supra. The recorded ECG and physiological telemetrycan even be downloaded wirelessly directly from a monitor recorder 14during monitoring while still snapped into the non-conductive receptacle25 on the electrode patch 15. The wireless interfacing enablesmonitoring to continue for an open-ended period of time, as thedownloading of the recorded ECG and physiological telemetry willcontinually free up onboard storage space. Further, wireless interfacingsimplifies patient use, as the patient (or caregiver) only need worryabout placing (and replacing) electrode patches 15 and inserting themonitor recorder 14. Still other forms of practical use of the extendedwear electrocardiography monitors 12 are possible.

The circuit trace and ECG electrodes components of the electrode patch15 can be structurally simplified. In a still further embodiment, theflexible circuit 32 (shown in FIG. 5 ) and distal ECG electrode 38 andproximal ECG electrode 39 (shown in FIG. 6 ) are replaced with a pair ofinterlaced flexile wires. The interlacing of flexile wires through theflexible backing 20 reduces both manufacturing costs and environmentalimpact, as further described infra. The flexible circuit and ECGelectrodes are replaced with a pair of flexile wires that serve as bothelectrode circuit traces and electrode signal pickups. FIG. 17 is aperspective view 180 of an extended wear electrode patch 15 with aflexile wire electrode assembly in accordance with a still furtherembodiment. The flexible backing 20 maintains the unique narrow“hourglass”-like shape that aids long term extended wear, particularlyin women, as described supra with reference to FIG. 4 . For clarity, thenon-conductive receptacle 25 is omitted to show the exposed batteryprinted circuit board 182 that is adhered underneath the non-conductivereceptacle 25 to the proximal end 31 of the flexible backing 20. Insteadof employing flexible circuits, a pair of flexile wires are separatelyinterlaced or sewn into the flexible backing 20 to serve as circuitconnections for an anode electrode lead and for a cathode electrodelead.

To form a distal electrode assembly, a distal wire 181 is interlacedinto the distal end 30 of the flexible backing 20, continues along anaxial path through the narrow longitudinal midsection of the elongatedstrip, and electrically connects to the battery printed circuit board182 on the proximal end 31 of the flexible backing 20. The distal wire181 is connected to the battery printed circuit board 182 by strippingthe distal wire 181 of insulation, if applicable, and interlacing orsewing the uninsulated end of the distal wire 181 directly into anexposed circuit trace 183. The distal wire-to-battery printed circuitboard connection can be made, for instance, by back stitching the distalwire 181 back and forth across the edge of the battery printed circuitboard 182. Similarly, to form a proximal electrode assembly, a proximalwire (not shown) is interlaced into the proximal end 31 of the flexiblebacking 20. The proximal wire is connected to the battery printedcircuit board 182 by stripping the proximal wire of insulation, ifapplicable, and interlacing or sewing the uninsulated end of theproximal wire directly into an exposed circuit trace 184. The resultingflexile wire connections both establish electrical connections and helpto affix the battery printed circuit board 182 to the flexible backing20.13

The battery printed circuit board 182 is provided with a batterycompartment 36. A set of electrical pads 34 are formed on the batteryprinted circuit board 182. The electrical pads 34 electrically interfacethe battery printed circuit board 182 with a monitor recorder 14 whenfitted into the non-conductive receptacle 25. The battery compartment 36contains a spring 185 and a clasp 186, or similar assembly, to hold abattery (not shown) in place and electrically interfaces the battery tothe electrical pads 34 through a pair battery leads 187 for powering theelectrocardiography monitor 14. Other types of battery compartment arepossible. The battery contained within the battery compartment 36 can bereplaceable, rechargeable, or disposable.

In a yet further embodiment, the circuit board and non-conductivereceptacle 25 are replaced by a combined housing that includes a batterycompartment and a plurality of electrical pads. The housing can beaffixed to the proximal end of the elongated strip through theinterlacing or sewing of the flexile wires or other wires or threads.

The core of the flexile wires may be made from a solid, stranded, orbraided conductive metal or metal compounds. In general, a solid wirewill be less flexible than a stranded wire with the same totalcross-sectional area, but will provide more mechanical rigidity than thestranded wire. The conductive core may be copper, aluminum, silver, orother material. The pair of the flexile wires may be provided asinsulated wire. In one embodiment, the flexile wires are made from amagnet wire from Belden Cable, catalogue number 8051, with a solid coreof AWG 22 with bare copper as conductor material and insulated bypolyurethane or nylon. Still other types of flexile wires are possible.In a further embodiment, conductive ink or graphene can be used to printelectrical connections, either in combination with or in place of theflexile wires.

In a still further embodiment, the flexile wires are uninsulated. FIG.18 is perspective view of the flexile wire electrode assembly from FIG.17 , with a layer of insulating material 189 shielding a bareuninsulated distal wire 181 around the midsection on the contact side ofthe flexible backing. On the contact side of the proximal and distalends of the flexible backing, only the portions of the flexile wiresserving as electrode signal pickups are electrically exposed and therest of the flexile wire on the contact side outside of the proximal anddistal ends are shielded from electrical contact. The bare uninsulateddistal wire 181 may be insulated using a layer of plastic, rubber-likepolymers, or varnish, or by an additional layer of gauze or adhesive (ornon-adhesive) gel. The bare uninsulated wire 181 on the non-contact sideof the flexible backing may be insulated or can simply be leftuninsulated. Both end portions of the pair of flexile wires aretypically placed uninsulated on the contact surface of the flexiblebacking 20 to form a pair of electrode signal pickups. FIG. 19 is abottom view 190 of the flexile wire electrode assembly as shown in FIG.17 . When adhered to the skin during use, the uninsulated end portionsof the distal wire 181 and the proximal wire 191 enable the monitorrecorder 14 to measure dermal electrical potential differentials. At theproximal and distal ends of the flexible backing 20, the uninsulated endportions of the flexile wires may be configured into an appropriatepattern to provide an electrode signal pickup, which would typically bea spiral shape formed by guiding the flexile wire along an inwardlyspiraling pattern. The surface area of the electrode pickups can also bevariable, such as by selectively removing some or all of the insulationon the contact surface. For example, an electrode signal pickup arrangedby sewing insulated flexile wire in a spiral pattern could have acrescent-shaped cutout of uninsulated flexile wire facing towards thesignal source.

In a still yet further embodiment, the flexile wires are left freelyriding on the contact surfaces on the distal and proximal ends of theflexible backing, rather than being interlaced into the ends of theflexible backing 20. FIG. 20 is a bottom view 200 of a flexile wireelectrode assembly in accordance with a still yet further embodiment.The distal wire 181 is interlaced onto the midsection and extends anexposed end portion 192 onto the distal end 30. The proximal wire 191extends an exposed end portion 193 onto the proximal end 31. The exposedend portions 192 and 193, not shielded with insulation, are furtherembedded within an electrically conductive adhesive 201. The adhesive201 makes contact to skin during use and conducts skin electricalpotentials to the monitor recorder 14 (not shown) via the flexile wires.The adhesive 201 can be formed from electrically conductive,non-irritating adhesive, such as hydrocolloid.

The distal wire 181 is interlaced or sewn through the longitudinalmidsection of the flexible backing 20 and takes the place of theflexible circuit 32. FIG. 21 is a perspective view showing thelongitudinal midsection of the flexible backing of the electrodeassembly from FIG. 17 . Various stitching patterns may be adopted toprovide a proper combination of rigidity and flexibility. In simplestform, the distal wire 181 can be manually threaded through a pluralityof holes provided at regularly-spaced intervals along an axial pathdefined between the battery printed circuit board 182 (not shown) andthe distal end 30 of the flexible backing 20. The distal wire 181 can bethreaded through the plurality of holes by stitching the flexile wire asa single “thread.” Other types of stitching patterns or stitching ofmultiple “threads” could also be used, as well as using a sewing machineor similar device to machine-stitch the distal wire 181 into place, asfurther described infra. Further, the path of the distal wire 181 neednot be limited to a straight line from the distal to the proximal end ofthe flexible backing 20.

An effective ECG compression solution can reduce battery powerconsumption, ameliorate storage restriction, and extend monitoring time.The effectiveness of an ECG compression technique is evaluated mainlythrough compression ratio, degree of error loss, and execution time. Thecompression ratio is the ratio between the bit rate of the originalsignal and the bit rate of the compressed one. The error loss is theerror and loss in the reconstructed data compared to non-compresseddata. The execution time is the computer processing time required forcompression and decompression. A lossless compressions may provide exactreconstruction of ECG data, but usually cannot provide a significantcompression ratio, thus may not be a good choice when high compressionratio is required. In addition, analysis of ECG data does not requireexact reconstruction; only certain feature of the ECG signal areactually important. Therefore, lossy compression, or techniques thatintroduce some error in the reconstructed data, is useful because lossycompression may achieve high compression ratios.

The ECG signal captured by the monitor recorder 14 is compressed by thecompression module 134 as part a firmware 132 located on microcontroller61 prior to being outputted for storage 135, as shown in FIG. 14 . Thecompression is performed using a plurality of compression algorithms,each of the compression algorithms including a series of stepsimplemented by the microcontroller 61. The compression algorithms arefurther described in detail below beginning with reference to FIG. 22 .

As further described below beginning with reference to FIG. 31 , thecompression module 134 can select the compression algorithms that arebest-suited for compressing the monitoring data of a particular patient.As further described below with reference to FIG. 31 , the compressionmodule 134 can further apply multiple compression algorithms to the sameECG data serially, or alternatively, the same compression algorithmmultiple times until the ECG data is compressed to a desired degree,which can be determined based on the randomness of the compressed data,with a high degree of randomness (and a low degree of entropy)indicating that significant further compression of the data is unlikelyto be achieved. Similarly, if after a round of compression the size ofthe data decreases only insignificantly, such as by 1% or 2%, remainsthe same, or increases, the compression module 134 performs no furthercompression.

In a further embodiment, the compression module can be implemented on amicrocontroller subcutaneous insertable cardiac monitor (ICM), such asone described in commonly-owned U.S. patent application Ser. No.15/832,385, filed Dec. 5, 2017, pending, the disclosure of which isincorporated by reference. FIG. 42 is a diagram showing an example of anICM 430 on which the compression module can be implemented. Briefly, theICM 430 implantable housing made of a biocompatible material that issuitable for implantation within a living body is provided. At least onepair of ECG sensing electrodes is provided on a ventral (or dorsal)surface and on opposite ends of the implantable housing operativelyplaced to facilitate sensing in closest proximity to the low amplitude,low frequency cardiac action potentials that are generated during atrialactivation. Electronic circuitry is provided within the housing assemblyincluding a low power microcontroller operable to execute under modularmicro program control as specified in firmware, an ECG front end circuitinterfaced to the microcontroller and configured to capture the cardiacaction potentials sensed by the pair of ECG sensing electrodes which areoutput as ECG signals, and a non-volatile memory electrically interfacedwith the microcontroller and operable to continuously store samples ofthe ECG signals. In a further embodiment, the memory of the ICM can be avolatile memory. The ICM 430 can be rechargeable, either using externalenergy sources specifically directed at recharging the ICM (such as viainductive charging or radiowave-based charging) or through internalenergy-harvesting capabilities, as described in U.S. Patent ApplicationNo. 62/870,506, filed Jul. 3, 2019, the disclosure of which isincorporated by reference. The rechargeable capabilities of the ICMallow for execution of even those compression algorithms that requiresignificant amount of electrical power. Still other kinds of cutaneousand implantable monitors on which the compression module (and hencecompression algorithms described below) can be implemented are possible.

FIG. 22 is a flow diagram showing a monitor recorder-implemented methodfor ECG signal processing and ECG data compressing for use in themonitor recorders of FIG. 4 . A series of ECG signals are sensed throughthe front end circuit 63, which converts analog ECG signals into anuncompressed digital representation. The compressing module 134 firstread the digital presentation of the ECG signals or ECG values (step201). The compressing module 134 subsequently encodes the ECG value(step 202). This encoding step achieves one level of compression and, inone embodiment, is a form of lossy compression, as further discussedinfra in FIG. 23 . The compressing module 134 also performs a secondlevel of compression by further encoding and compressing the sequence ofcodes resulting from the encoding process of step 202 (step 203). Thecompressed data is stored into a non-volatile memory, such as the flashmemory 62. In a further embodiment, the memory in which the compresseddata is stored can be a volatile memory.

Monitoring ECG (step 201) is described in FIG. 12 . Encoding ECG values(step 202) is performed by translating each sample data into one ofcodes, or encodings, further described with reference to the table inFIG. 24 . By encoding ECG data in the form of a series of codes, a levelof compression is achieved. FIG. 23 is a flow diagram showing a monitorrecorder-implemented method for encoding ECG values. FIG. 24 is anexample of a panel of codes or encodings with each code covering a rangedefined by a lower threshold ECG value and an upper threshold ECG value,to be referenced to during the encoding process described in FIG. 23 .In one embodiment, a series of ECG values are obtained, which constitutea datastream (step 211). The series of ECG value can be one of rawelectrocardiography value, processed electrocardiography value, filteredelectrocardiography value, averaged electrocardiography value, orsampled electrocardiography value. The compression module 134 defines aplurality of bins, each bin comprising a lower threshold ECG value, anupper threshold ECG value, and an encoding or code (step 212). Oneexample of such a panel of the bins is shown in the Table in FIG. 24 ,with the first column denoting the lower threshold ECG value, the secondcolumn denoting the upper threshold ECG value, and the third columndenoting the code of a bin. An ECG data value is assigned to acorresponding bin, based upon the difference between the data value anda serial accumulator. The first serial accumulator is set to apre-determined value such as a center value of an ECG recorder (step213), each succeeding serial accumulator is a function of a previousserial accumulator and the actual ECG reading and will be describedinfra. For the series of the ECG values, the following encoding stepsare performed by the compression module (steps 214 to 220). These stepsincludes: selecting the ECG value next remaining in the series to beprocessed (step 215); taking a difference between the selected ECG valueand the serial accumulator (step 216); identifying the bin in theplurality of the bins corresponding to the difference (step 217), whichwill be further described infra; representing the selected ECG value bythe encoding for the identified bin (218); and adjusting the serialaccumulator by a value derived from the identified bin (step 219).Through this process, each ECG value is represented, or encoded, by oneof the bins. As a result, one level of data compression is achievedsince the limited number of bins requires less storage space compared tothe actual ECG data values.

Several ways of executing the step 217, i.e., identifying the bin in theplurality of the bins corresponding to the difference between theselected ECG value and the serial accumulator, or assigning a differencebetween the selected ECG value and the serial accumulator to a properbin. In one embodiment, a difference is assigned to a bin when thedifference lies between the lower threshold ECG value and the upperthreshold ECG value of the bin. There are two options to assign a binwhen a difference between the selected ECG value and the serialaccumulator falls onto the lower threshold ECG value or the upperthreshold ECG value of a bin. In one option, a bin is identified whenthe difference is equal to or larger than the lower threshold ECG valueand smaller than the upper threshold ECG value of the identified bin. Inthe other option, a bin is identified when the difference is larger thanthe lower threshold ECG value and equal to or smaller than the upperthreshold ECG value of the identified bin.

During the step 219, the value derived from the identified bin can bethe lower threshold ECG value, the higher threshold ECG value, or anumber derived from the lower threshold ECG value, upper threshold ECGvalue, or both. The derivation can be an addition or subtraction of thelower or upper threshold ECG value by a constant number or an offset.The derivation can also be an adaptive process wherein the offset may beadjusted to input ECG data, and varies from one bin to another bin.

Converting ECG values into a limited numbers of codes facilitate afurther compression step which will be described infra. Some data errorloss is introduced by the encoding process; however, proper bin setupminimizes the ECG data error loss and preserves useful data essentialfor accurate diagnosis, including P-wave signal. The number of codes andthe lower and upper threshold ECG value of the codes are determined toachieve both efficient encoding and sufficient data reconstruction,especially for P-wave signals. The number of codes and the lower andupper threshold ECG value of the codes are flexible and can be adjustedto adapt to ECG data input and storage space. In one embodiment, thenumber of the bins are chosen from 2³ to 2¹⁰. A higher number of binsusually results in less ECG data error loss but more storage space andbattery power use.

The proper demarcation of upper and lower thresholds also reduces errorloss and contributes to accurate re-construction of ECG value and graphshape. The number of bins and the thresholds for these bins arecarefully selected to keep essential information of the ECG signals andfilter away non-essential information, with a special emphasis toaccurately representing the P-wave. Normally, each successive bincontinues forward from a previous bin so as to cover a contiguous rangeof electrocardiography values (also referred to as electrode voltagevalues). In one embodiment, the size of the bins, i.e., the intervalbetween the higher threshold ECG value and the lower threshold ECGvalue, are not equal thought the contiguous range; instead, areas ofhigh frequency calls for a smaller size of bins. The size of the bins ispartly determined by the frequency of the ECG values falling into thebin.

In one embodiment, 2⁴=16 bins are used, as described with reference tothe table in FIG. 24 where the lower threshold ECG value and upperthreshold ECG value for each bin are also provided. This setup providesminimum error loss and a significant compression ratio, among otherconsiderations. The first, second, and third columns represent the lowerthreshold ECG value, the upper threshold ECG value, and the coding ofthe bins. The bin that an ECG data will fall into depends on thedifference between the raw ECG data value and corresponding serialaccumulator compared to the range that the bin covers. If an ECG rawdata falls into a particular bin, the raw ECG data can be represented bythe code of the bin. In this example, the codes are encoded with afour-bit storage space, with one bit to encode sign and three bits toencode magnitude. Similar, up to 32 codes can be encoded with a five-bitstorage space, with one bit to encode sign and 4 bits to encodemagnitude.

The minimum (Min) and maximum (Max) values in the table in FIG. 24defines an inclusive range of ECG values for each ECG code. An input ECGvalue that falls within the range defined by a pair of Min and Maxvalues is encoded by the code appearing in the third column in thetable. The Min and Max ranges can be the same for all of the bins or canbe tailored to specific ranges of ECG values, to emphasize higher orlower density. For example, the range of Min and Max values 5,001-50,000corresponding to code +7 is low density and reflects the expectationthat few actual ECG values exceeding 5001 μV will occur. As a furtherexample, the Min and Max ECG value ranges can be evenly definedthroughout, or be doubled in each of the successive bins. In oneembodiment, the number of bins is selected to be a power of two,although a power of two is not strictly required, particularly when asecond stage compression as further described below with reference toFIG. 26 . In a further embodiment, the density of the Min and Max valuecan be adjusted to enhance ECG signal detection, such as the P-wavesignal, as further described infra beginning with reference to FIG. 28 .

FIG. 25 is an example illustrating the encoding and compression schemein accordance with method and parameters as described with reference toin FIGS. 23 and 24 . The first three ECG values of an ECG datastream,12000, 11904, and 12537, are shown in column I to show a recursiveprocess. Remaining values are omitted since they are processed throughthe same recursive process. The initial ECG value, 12000, is equivalentto the center value of the ECG recorder. The initial serial accumulatoris assigned to the center value of the ECG recorder, 12000. Thedifference between the initial ECG value to the initial serialaccumulator is 0, which falls within the lower and upper threshold ofbin 0. Thus the initial ECG value is encoded with the code 0. 12000 istransferred to next row as the serial accumulator for next ECG value.The next ECG value is 11904. The difference between the next ECG valueand the serial accumulator for the second value is 11904−12000=−96. Thedifference of −96 falls into the bin with the code of −3, where thelower threshold of the bin is −41 and the upper threshold of the bin is−150. Thus, the second ECG value is encoded with the code of −3, whichis the bin identification. For the purpose of decoding the second value,an encoder first refers to the assigned bin, which is bin −3; theencoder then reads the lower threshold ECG value of the assigned bin −3,which is −41; and the encoder finally add the lower threshold ECG valueof the assigned bin to the decoded value of the first ECG value, whichis 12000, to arrive at a decoded value of 11959. The decoded value 11959in turn serves as the serial accumulator for the next ECG value, in thiscase the next ECG value is the third one of 12537. The differencebetween the third value and its corresponding serial accumulator is12537−11959=578. This difference, 578, falls into the bin with a code of+5, which has a lower threshold ECG value of 301 and upper threshold ECGvalue of 1500. Thus the third ECG value is encoded with the code of +5.The third ECG value is decoded by adding the lower threshed ECG value ofthe assigned bin +5, which is 301, to the decoded value of second ECGvalue, which is 11959, to arrive at the decoded value of 12260. Thedecoded value of 12260 in turn will serve as the serial accumulator forthe next ECG value. The encoding process continue until the last readingis taken. The encoder keeps track of the accumulated encoded value asthe encoding process progresses along.

The encoding process is also a lossy compression process that encodesraw ECG signals with a finite number of codes. This process capturesessential information while achieving significant data compression. Inone embodiment, another compressing step is performed. The othercompression step may be performed independently. The other compressionstep may also be performed on top of the encoding process describedabove to achieve a higher level compression than one step alone. Thesecond compression step can be a lossless compression performed on thecodes from the first step. In one embodiment, the compression ratio ofthe second compression is in the range of 1.4 to 1.6, increasing thedata storage capacity of a non-volatile memory by more than 41-66%. Inanother embodiment, the compression ratio of the second compression isin excess of 1.6, increasing the data storage capacity of a non-volatilememory by more than 66%. Thus, the combination of the lossy compressionand the lossless compression serves to achieve both high fidelity of theECG signal preservation and high compression ratio, which translate intoincreased data storage capacity and reduced power consumption for theambulatory electrocardiography monitor, resulting in extended wear timeof the monitor.

In one embodiment, the second compression is effected by encoding asequence of codes obtained from the first compression into a singlenumber between 0 and 1, with frequently used codes using fewer bits andnot-so-frequently occurring codes using more bits, resulting in reducedstorage space use in total. FIG. 26 is a flow diagram showing a monitorrecorder-implemented method for further compressing the codes. Asequence of the codes corresponding to the series of the ECG values isprovided to the compressing module 134. The compressing module 134 set arange of 0 to 1 to an initial sequence of the codes (step 231). Thecompressing module 134 further performs recursive steps of assigningeach successive codes into a sub-range within a previous range accordingto the probabilities of the codes appearing after a code (steps232-239). The compressing module 134 obtains an estimation ofprobabilities of next codes, given a current code (step 233). Severalvariations of calculating and adjusting the probabilities of the nextcodes will be described infra. The compressing module 134 divides therange of the current code into sub-ranges, each sub-range representing afraction of the range proportional to the probabilities of the nextcodes (step 234). These sub-ranges are contiguous and sequential. Thecompressing module 134 reads the next code (step 235) and selects thesub-range corresponding to the read next code (step 236). The read nextcode is represented, or encoded, by the corresponding sub-range (step237). The sub-range corresponding to the read next code is assigned tobe the range for the code next to the read next code (step 238), and therange is further divided into sub-ranges with each sub-rangerepresenting a fraction of the range proportional to the probabilitiesof codes next to the read next code (step 39). In this way, each code inthe sequence of the codes is represented by, or encoded through, itslocation within a sub-range through a recursive process. During therecursive process, strings of codes represented by the selectedsub-ranges are encoded into part of the single number between 0 and 1and can be periodically or continually stored into the non-volatilememory, can be stored on-demand or as-needed, or can be queued up andstored en masse upon completion of the process. One example of thenon-volatile memory is the flash memory 62. In a further embodiment, thememory into which the encoded subranges are stored could also be avolatile memory.

The compressing module 134 uses a statistical model to predict what thenext code is, given a current encoding (step 233). In one embodiment, atotal of 16 codes or bin numbers are used, thus the statistical modeluses 16 tables, one for each current code. Within each table, numericpossibilities for 16 possible next codes given the particular currentcode are generated. In one embodiment, the probabilities of the nextcodes can be calculated from sample ECG values. In another embodiment,the probabilities of the next codes can be modified by ECG dataincluding recorded ECG data and data presently recorded. In stillanother embodiment, the probabilities of next codes can be adaptive,i.e, adjusted or varied along the recursive compression steps. Finally,in yet another embodiment, the compressing module 134 may use astatistical model to arrive at the estimation of probabilities of nextcodes, given two or more consecutive preceding codes. When twoconsecutive preceding codes are used, 16×16=256 different pairs ofconsecutive codes are possible. The compressing module 134 generates 256tables, each tables containing numeric possibilities for 16 possiblenext codes given a particular pair of previous codes. When threeconsecutive preceding codes are used, 16×16×16=4096 different trios ofconsecutive codes are possible. The compressing module 134 generates4096 tables, each tables containing numeric possibilities for 16possible next codes given a particular trio of previous codes. Using twoor more consecutive preceding codes further enhances compression ratiocompared to using one preceding code, but also demands more processingpower from the microcontroller.

In some situations, minimum amplitude signals may be masked by or becomeindistinguishable from noise if the granularity of the encoding is toocoarse. Such overly inclusive encoding can occur if the Min and Max ECGvalues in the table in FIG. 24 for an ECG code define a voltage rangethat is too wide. The resulting ECG signal will appear “choppy” anduneven with an abrupt (and physiologically inaccurate and potentiallymisleading) slope. FIG. 28 is a graph 280 showing, by way of example, anECG waveform 281 with a low amplitude signal that has been degraded bycompression artifacts. The amplitude 283 of the P-wave 282 issignificantly smaller than the amplitude 285 of the QRS-complex 281. Inaddition, the QRS-complex 281 has a pronounced slope with a relativelysmooth peak and valleys. Due to compression artifacts 256 caused by anover-wide range of the Min and Max ECG values used in encoding, theP-wave 282 has a stepped appearance. As a result, important signalcontent is lost due to too few values being used to encode the P-wave282.

In a further embodiment, the density of the Min and Max values can beadjusted to optimize cardiac signal detail capture by enhancing lowamplitude ECG signal detail, especially as implicit in the shape of theslope of the P-wave. FIG. 29 is a flow diagram showing a routine 290 forproviding rescalable encoding. Rescaling is generally performed duringdata compression values (step 202 in FIG. 24 ) as raw ECG signals arebeing stored in a monitor recorder's memory. As initial steps, aduration for each temporal window and an acceptable peak-to-peak voltagethreshold are respectively defined (steps 291 and 292). In oneembodiment, a temporal window duration of about five seconds, whichcould be plus or minus some amount, and an acceptable peak-to-peakvoltage threshold of about 1.0 mV are used, although other temporalwindow durations and acceptable peak-to-peak voltage thresholds arepossible. The raw ECG signal is then iteratively processed assuccessively-occurring temporal windows (steps 293-296). For eachtemporal window (step 293), the peak-to-peak voltage is computed (step294). If the peak-to-peak voltage is less than the acceptable threshold(step 295), the encoding table and acceptable peak-to-peak voltagethreshold are rescaled by a factor of 0.5 (step 296). Other scalingfactors are possible. ECG signal processing continues with the newencoding (step 297).

In addition, if, while encoding samples (step 202 in FIG. 24 ), thedesired difference between the peak-to-peak voltage and the acceptablepeak-to-peak voltage threshold exceeds the Max value in the entry in thetable in FIG. 24 for the ECG code to which the ECG value being encodedcorresponds, the encoding table and acceptable peak-to-peak voltagethreshold are rescaled by a factor of 2.0, until the Max value in thattable entry is larger than the ECG value being encoded. Other scalingfactors are possible. When the table is scaled in this way, a message isinserted into the data indicating how the table has been scaled, whichcan be used during decompression and post-processing.

Flexible application of multiple compression algorithms optimizedefficient data compression and adaptable to a particular patient allowsa cardiac monitor to compress ECG monitoring data to a degree sufficientto enable long-term monitoring while preserving the features importantfor creating the diagnosis. FIG. 31 is a flow diagram showing a method310 for flexible ECG data compression in accordance with one embodiment.The method can be applied on the monitor recorder 14 described above,though application on other cutaneous or implantable monitors is alsopossible. For example, the method can be implemented on a subcutaneousinsertable cardiac monitor (ICM), such as one described incommonly-owned U.S. patent application Ser. No. 15/832,385, filed Dec.5, 2017, abandoned, the disclosure of which is incorporated byreference.

ECG monitoring is performed by a cardiac monitor, producing a series ofelectrocardiography voltage values that are recorded using theelectrodes of the monitor (also referred to as electrode voltages in thedescription below) (step 311). One or more of compression algorithms areselected by the microcontroller of monitor for compressing the series ofthe electrode voltages as further described below with reference toFIGS. 32-34 (step 312). The selected compression algorithms are toapplied to at least a portion of the series of the electrode voltages,as further described below beginning with reference to FIG. 35 (step313). Whether the necessary degree of compression of the at least theportion of the series of the electrode voltages has been achievedthrough the effectiveness of the compression algorithms being applied isevaluated (step 314). As described above, the necessary degree ofcompression could be evaluated based on the randomness of the compresseddata, which could be compared to a threshold to determine whether thedesired degree of compression has been achieved. Alternatively, thecompression ratio achieved during the application of the algorithmscould be evaluated to determine is further compression of the electrodevoltage series is likely through further application of additionalcompression algorithms. For example, if after a round of compression thesize of the data decreases only insignificantly, below a presetthreshold, decreasing by 1% or 2% or remains the same or even increases,the desired degree of compression is determined to having been achieved.Still other ways to determine whether the desired degree of compressionhas been achieved are possible.

If the desired degree of compression has not been achieved (step 314),the method returns to step 312. If the desired degree of compression hasbeen achieved (step 314), the method 310 ends.

Determining which compression algorithms are best-suited for compressingparticular ECG monitoring data informs selection of an algorithm. FIG.32 is a flow diagram showing a routine 320 for selecting compressionalgorithms for use in the method 310 of FIG. 31 in accordance with oneembodiment. If there are already results of compression using analgorithm (or algorithms) previously selected and applied duringprevious iterations of the method 310 (step 322) on this patient, theroutine moves to step 322, where a selection scheme is chosen based onthe result (step 322). As described below, the monitor could utilize twoschemes to select a compression algorithm for use. If one of the schemeswas used in the immediate previous iteration of the routine 320 and theeffectiveness of the selected algorithms (evaluated based on a criteriasuch a degree of compression achieved) were below a desired threshold,the scheme that was not used in the immediate previous iteration 320 isselected for current application. Likewise, if the results are availableand the previously-used selection scheme selected compression algorithmswith a satisfactory effectiveness, the same selection scheme can bechosen. One of the selection schemes involves selecting one or more ofthe compression algorithms based on the characteristics of the waveformswithin a segment of the series of electrode voltage values beingcompressed, as further described with reference to FIG. 33 . The secondselection scheme involves testing a plurality of the compressionalgorithms are first tested on a segment of the series of the electrodevoltage values that needs to be compressed and the compressionalgorithms are selected based on the test, as further described withreference to FIG. 34 , ending the routine 320. The chosen selectionscheme is performed to select compression algorithms for subsequentapplication (step 323), ending the routine 323.

If the results of previous application of the algorithms are notavailable (321), then both the characteristic-based selection scheme andthe test-based selection scheme are performed (steps 324, 325), asfurther described below with reference to FIGS. 33 and 34 respectively.The compression algorithms chosen by both schemes are tested (step 326)on a segment of the series of electrode voltage values (which could be adifferent segment from the one used for empirical testing in step 325)and the compression algorithm (or algorithms) with greatest compressioneffectiveness (or based on other criteria) is chosen for application(step 327), ending the routine 320.

Compression efficiency for a given algorithm tends to vary greatlythroughout the recording due to changes in shapes of the ECG waveformsbeing compressed and the compatibility of the shapes with the algorithmperforming the compression. FIG. 33 is a flow diagram showing a routine330 for selecting compression algorithms based on characteristics of thewaveforms within the series for use in the routine of FIG. 32 .Initially, characteristics of ECG waveforms within the series of therecorded ECG values is determined (step 331). Determining suchcharacteristics can include analyzing frequency content of the waveformsand performing spectrum analysis of the waveforms. Based on thedetermined characteristics, suitable compression algorithms areselected, ending the routine 330 (step 332). For example, if a waveformhas high frequency content—often indicative of noise, a lossycompression algorithm may be best suited for compressing that waveform.Likewise, if a waveform has mostly low frequency content, a non-lossyalgorithm can be employed for compression to preserve as much detail aspossible. Similarly, the determined characteristics can include theheart rate experienced by the patient, and a compression algorithm canbe determined based on the corresponding heart rate. For example, if theanalyzed waveforms indicate that the patient has a high rate, arun-length encoding compression algorithm can be selected, though othercompression algorithms could also be selected. Likewise, if the analyzedwaveforms indicate that the patient has a low heart rate, compressionalgorithms such as difference encoding can be applied, though othercompression algorithms could also be applied.

Suitable compression algorithms can also be chosen based on an empiricaltest. FIG. 34 is a flow diagram showing a routine 340 for selecting acompression algorithm based on empirical testing for use in the routine320 of FIG. 32 in accordance with one embodiment. Initially, criteriafor evaluating the result of compression of the series of ECG values isselected (step 341). Such criteria can be an amount of battery powerthat is consumed by the algorithm, as well as the amount of memory spacethat is occupied by the results of the compression. Other criteria arepossible. Each of the available compression algorithms is then appliedto a segment of the series of ECG values that needs to be compressed(step 342). The results of the compression by each of the algorithms areanalyzed against the criteria (step 343), and one or more of thealgorithms whose results are most fitting of the criteria (such asconsuming least battery power or creating the greatest degree ofcompression) are selected for compression (step 344), ending the routine340.

The selected algorithms are applied to the series of the ECG values thatneeds to be compressed. FIG. 35 is a routine 350 for applyingcompression algorithms to a series of ECG values for use in the method310 of FIG. 31 . Initially, segments of the ECG values that includenoise are identified, and noise is removed using run-length encoding:representing noise within the encoding by one or more symbols thatrepresent the length of the noise. For example, a segment of noiselasting five sections can be represented by a symbol “J5,” though otherrepresentations are also possible (step 351). Once the noise iscompressed, one or more of the steps 352-356 can be optionallyperformed, with the results being stored in a non-volatile memory or avolatile memory of the cardiac monitor performing the compression. Whilethe algorithms implemented in steps 352-356 are listed in a particularorder, the algorithms could be applied serially in a different sequence.Optionally, an average waveform compression algorithm can be applied, asfurther described below with reference to FIG. 36 (step 352).Optionally, a modified waveform average compression algorithm can beapplied, as further described below with reference to FIG. 37 (step353). Optionally, a complex deconstruction and modeling encodingalgorithm can be applied, as further described below with reference toFIG. 38 (step 354). Optionally, a learning algorithm-based compressionis applied, as further described below with reference to FIG. 39 (step355). Optionally, dynamic-learning-algorithm-based compression can beperformed, as further described below with reference to FIG. 40 (step356), ending the routine 350. The optional steps 352-356 can beperformed in any combination with each other to compress the series ofECG values to a desired extent. Further, while at least one of thealgorithms implemented by steps 352-356 is applied, the appliedalgorithm could be the only algorithm being applied during the iterationof the routine 350.

Each cardiac waveform subset (also referred to as a “beat” in thedescription below) can be represented as a set of points, each of thepoints associated with a magnitude, and therefore mathematicaloperations can be performed with the cardiac waveforms. As most cardiacwaveforms of a healthy individual are likely to be similar to eachother, memory space can be saved by storing a difference between awaveform and an average of waveforms similar to that waveforms ratherthan the waveform in the entirety. FIG. 36 is a flow diagram showing aroutine 360 for an application of waveform average compression algorithmfor use in the routine 350 of FIG. 35 . A beat is detected (step 361)and compared to a previously-detected beats for the patient (step 362).If the beat matches a previous beat, the two beats are averaged (step365). The similarity is evaluated based on analyzed characteristics ofthe waveforms, such as width and height of waves within the waveform,and if a preset level of similarity between the characteristics is met,the waveforms are determined to be a match. The sufficient number ofsimilar waveforms is also preset, though in a further embodiment, otherways to set the number of similar waveforms is possible. In oneembodiment, only the number of similar waveforms within the series ofECG values is used for determining the sufficiency of the detectedwaveforms. In a further embodiment, similar waves detected from otherpatients or from the same patient during a different monitoring sessioncan be considered for similarity and averaged.

If beat does not match to a previously detected beat (or this beat isthe first beat being analyzed) (step 364), a stored waveform templatesimilar to the new beat being processed is created (step 364).

The average of the two beats (if available) or the newly createdtemplate (if no sufficient number of similar waveforms have beenidentified) are subtracted from the waveform being processed (366), andthe result of the subtraction is stored as a compression for the ECGvalues that comprise the beat being processed (step 367). If the cardiacmonitoring is ongoing, the routine 360 returns to step 361. If themonitoring has stopped, the routine 360 ends. While the description ofthe routine 360 references averaging entire cardiac waveforms from theentire waveforms and subtracting the average for the entire waveform, ina further embodiment, in the routine 360, the operations could beperformed to portions of the cardiac waveforms. For example, a QRScomplex could be averaged with similar QRS complexes and the average QRScomplex could be subtracted from the QRS complex being compressed, withthe result of the subtraction being stored as the result of thecompression of that QRS complex. Likewise, a template could beidentified only for the QRS complex, with subsequent processinginvolving only the QRS complex.

The characteristics of ECG waveforms may change depending on theactivity level of the patient, and the compression of the waveformscould be adjusted in light of the activity level. FIG. 37 is a flowdiagram showing a routine 370 for an application of waveform averagecompression algorithm for use in the routine 360 of FIG. 35 inaccordance with one embodiment. E A beat is detected (step 371) andcompared to previously-detected beats (step 372). If the beat matchesone of the previously detected beats (step 373), the two beats areaveraged (step 375). The similarity is evaluated based on analyzedcharacteristics of the waveforms, such as width and height of waveswithin the waveform, and if a preset level of similarity between thecharacteristics is met, the waveforms are determined to be similar. Thesufficient number of similar waveforms is also preset, though in afurther embodiment, other ways to set the number of similar waveforms ispossible. In one embodiment, only the number of similar waveforms withinthe series of ECG values is used for determining the sufficiency of thedetected waveforms. In a further embodiment, similar waves detected fromother patients or from the same patient during a different monitoringsession can be considered for similarity and averaged.

If a sufficient number of similar waveforms have not previously beendetected (step 373), a waveform template similar to the waveform beingprocessed is created (step 374).

The average, if calculated, or the template, are subsequently modifiedto best fit the detected waveform (step 376). Thus, for example if apatient is exercising when a waveform is recorded, and has a shorterwaveform, the average waveform or the template are compressed to bestfit the detected waveform being processed. On the other hand, if thepatient was sedentary when the waveform is detected and the waveform islonger due to decreased heart rate, the average waveform or the templateare stretched to best fit the detected waveform.

The modified average of the similar waveforms (if available) or themodified template (if no sufficient number of similar waveforms havebeen identified) are subtracted from the waveform being processed (step377), and the result of the subtraction is stored as a compression forthe ECG values that comprise the waveform being processed (step 379).Optionally, further compression may be applied to the result of thesubtraction prior to the storage (step 378). If the cardiac monitoringis ongoing, the routine 370 returns to step 371. If the monitoring hasstopped, the routine 370 ends. While the description of the routine 370references averaging entire cardiac waveforms from the entire waveformsand subtracting the average for the entire waveform, in a furtherembodiment, in the routine 370, the operations could be performed toportions of the cardiac waveforms. For example, a QRS complex could beaveraged with similar QRS complexes, the average could be modified tobest fit to the QRS complex being processed, and the average QRS complexcould be subtracted from the QRS complex being compressed, with theresult of the subtraction being stored as the result of the compressionof that QRS complex.

Portions of a cardiac waveform can be represented by a mathematicalformula and parameters of the formula can be stored to save memory spaceinstead of storing the entire portion of the waveform represented by theparameters. FIG. 38 is a flow diagram showing a routine 390 for anapplication of waveform average compression algorithm for use in theroutine 350 of FIG. 35 in accordance with one embodiment. A beat isdetected within the series of ECG values (step 391) and separated intosegments, such as individual waves within the waveforms (such as the P,Q, R, S, T, and U waves) (step 392). The individual segments arerepresented by a mathematical model and one or more parameters for thatmodel. For example, an R-wave can be described by 5 data points (start,low, high, end, and duration of the wave), with the parameters being thevalues of the data points (time and amplitude). A P-wave can berepresented as a half-circle, with the amplitude of the circle being theparameter. The ST segment can be as two sine waves, with the parametersincluding a point of time of the start of the waves, the amplitude ofthe waves and the phase of the waves. Other waves can similarly berepresented through trigonometric or algebraic formulas. Still otherrepresentations are possible.

Optionally, each segment is reconstructed using the mathematical modelof that segment and the parameters of that model, and the reconstructedsegment is subtracted from the segment that served as the basis for themathematical model representation (step 394). Thus, for example, if asegment includes a P-wave, a P-wave reconstructed using the model andparameter used to represent the P-wave in the identified in the segmentis subtracted from the P-wave identified in the segment.

The representation obtained in step 393, or step 394 was performed, theresult of the subtraction (as well as the parameters the parametersused), is stored as the result of the compression of that particularsegment (step 395). If the cardiac monitoring is ongoing, the routine390 returns to step 391. If the monitoring has stopped, the routine 390ends.

Compression algorithms using an encoding table generally use a tablethat is fixed, such as shown above with reference to FIG. 24 , and whichmay not always be appropriate for a particular current patient. Theappropriateness of a particular compression algorithm can be increasedby training a learning algorithm to adjust the encoding table usedduring compression. FIG. 39 is a flow diagram showing a routine 400 forapplying a learning-algorithm-based compression in accordance for use inthe routine 350 in accordance with one embodiment. A compressionalgorithm that uses an encoding table is applied to a portion of theseries of ECG values that are being processed for at least a portion ofa training period before the routine 400 moves to step (step 401). Forexample, if a cardiac monitoring is scheduled to take place for 14 days,the training period could be one day. The effectiveness of thecompression of the portion of the series is evaluated, such as based onthe amount of memory space required to store the compressed values (andconsequently, degree of compression of the portion of the series)(step402). If the effectiveness is satisfactory (such as meeting a predefinedthreshold), and the training period has not yet been completed (step405), the routine returns to step 401. If the effectiveness is notsatisfactory (step 403), the encoding table is revised, as furtherdescribed below with reference to FIG. 41 below, and the routine 400returns to step 405. If the training period has been completed (step405), the ECG values obtained during the remaining period are compressedwith the algorithm using the table that has either been revised or thathas a satisfactory effectiveness (step 406), ending the routine 400. Ifthe training period has not been completed (step 405), the routine 400returns to step 401.

The evaluation of the effectiveness of an encoding table used by acompression algorithm can also be performed continually and adjustedthroughout the entire physiological monitoring period. FIG. 40 is a flowdiagram showing a routine 410 for applying adynamic-learning-algorithm-based compression in accordance for use inthe routine 350 in accordance with one embodiment. An encoding algorithmusing an encoding algorithm is used to compress a portion of the seriesof the ECG values for a time period, such as for a few hours, thoughother time periods could be used (step 411). The effectiveness of thecompression of the portion of the series is evaluated, such as based onthe amount of memory space required to store the compressed values (step412). If the effectiveness is satisfactory (such as meeting a predefinedthreshold), and unprocessed data remains (step 415), the routine returnsto step 411. If the effectiveness is not satisfactory (step 413), theencoding table is revised (step 414), as further described below withreference to FIG. 41 below, and the routine 410 returns to step 415,with any further processing being done using the revised table. If theeffectiveness is satisfactory (step 413) and no data uncompressed usingthe algorithm remains, the routine 410 ends.

Revising the encoding table allows to optimize the table for the patientwhose data is being recorded. FIG. 41 is a flow diagram showing aroutine 420 for revising the encoding table for use in the routines 400,410 of FIGS. 39 and 40 . The frequency of use of each of the entries inthe table is evaluated (step 421). Entries whose use falls below apredefined frequency are removed from the table (step 422). Additionalentries are created based on the evaluation and the removed entries,ending the routine 420. For example, if multiple entries spanning acertain range of ECG values were not used with a great enough frequencyduring the compression, a single entry covering all of the ranges of ECGvalues of the removed entries is created.

While the compression algorithms above are described as being applied toelectrocardiology data, in a further embodiment, at least some of thecompression algorithms could be applied to other types of physiologicaldata.

While the invention has been particularly shown and described asreferenced to the embodiments thereof, those skilled in the art willunderstand that the foregoing and other changes in form and detail maybe made therein without departing from the spirit and scope.

What is claimed is:
 1. A subcutaneous electrocardiography monitorconfigured for test-based data compression, comprising: an implantablehousing for implantation within a patient; a plurality ofelectrocardiographic (ECG) sensing electrodes; electronic circuitryprovided within the housing, the electronic circuitry comprising: an ECGfront end circuit interfaced to a microcontroller and configured tosense electrocardiographic signals via the ECG sensing electrodes; amemory; and a microcontroller operable to execute under modular microprogram control as specified in firmware, the microcontroller configuredto: obtain a series of electrode voltage values based on the sensedelectrocardiographic signals; use a plurality of selection schemes tochoose one or more of a plurality of compression algorithms associatedwith each of the selection schemes for testing, wherein one of theselection schemes comprises selecting the one or more compressionalgorithms based on characteristics of at least some waveforms withinthe electrode voltage series and another one of the selection schemescomprises performing empirical testing of the plurality of thecompression algorithms and selecting the one or more compressionalgorithms based on the empirical testing; test the selected one or morecompression algorithms comprising applying the one or more compressionalgorithms chosen using each of the selection schemes to a segment ofthe electrode voltage series; analyze results of the testing comprisingcomparing the results of the test achieved using the one or morecompression algorithms using each of the selection schemes; select theone or more compression algorithms chosen using one of the selectionschemes for compressing at least a portion of the electrode voltageseries based on the analysis; obtain a compression of at least theportion of the electrode voltage series by applying the one or morecompression algorithms selected based on the analysis to at least theportion of the electrode voltage series; and store the compressionwithin the memory.
 2. A monitor according to claim 1, wherein thecharacteristics of each of the waveforms comprise one or more of afrequency of content associated with that waveform and a heart rateassociated with that waveform.
 3. A monitor according to claim 1,wherein the another selection scheme comprises setting one or morecompression criteria, applying each of the plurality of the compressionalgorithms to a part of the series of electrocardiography values,analyzing a result of compression of the part by each of the algorithmsusing the compression criteria, wherein the one or more of thecompression algorithms are set as chosen by the another selection schemebased on the analysis of the result of the compression of the part.
 4. Amonitor according to claim 3, wherein the compression criteria compriseone or more of an amount of the battery amount of battery power consumedby one of the algorithms and an amount of memory space that is occupiedby the results of the compression of the part using one of thealgorithms.
 5. A monitor according to claim 1, further comprising:determine an availability of a result of compression of a furtherelectrode voltage series using the one or more algorithms selected usingone of the selection scheme and the another selection scheme, whereinthe testing is performed in the absence of the result of compression ofthe further electrode voltage series.
 6. A monitor according to claim 1,wherein the one or more compression algorithms selected based on theanalysis comprise a plurality of the compression algorithms and theselected algorithms are serially applied to at least the portion of theelectrode voltage series.
 7. A monitor according to claim 1, themicro-controller further configured to identify noise within theelectrode voltage series, wherein the one or more compression algorithmsselected based on the analysis are not applied to at least some of theidentified noise.
 8. A monitor according to claim 7, themicro-controller further configured to represent the noise with at leastone symbol within the compression.
 9. A monitor according to claim 1,wherein the one or more compression algorithms selected based on theanalysis comprise a single one of the compression algorithms and thesingle compression algorithm is applied multiple times to at least theportion of the electrode voltage series.
 10. A cutaneouselectrocardiography monitor configured for test-based data compression,comprising: a housing configured to fit within a receptacle on a patchapplied to a patient; an electrocardiographic front end circuit withinthe housing that is operable to sense electrocardiographic signalsthrough electrocardiographic electrodes, each electrocardiographicelectrode positioned on one end of the electrode patch: a memory withinthe housing; and a micro-controller within the housing operable toexecute under a micro-programmable control and configured to: obtain aseries of electrode voltage values based on the sensedelectrocardiographic signals; use a plurality of selection schemes tochoose one or more of a plurality of compression algorithms associatedwith each of the selection schemes for testing, wherein one of theselection schemes comprises selecting the one or more compressionalgorithms based on characteristics of at least some waveforms withinthe electrode voltage series and another one of the selection schemescomprises performing empirical testing of the plurality of thecompression algorithms and selecting the one or more compressionalgorithms based on the empirical testing; test the selected one or morecompression algorithms comprising applying the one or more compressionalgorithms chosen using each of the selection schemes to a segment ofthe electrode voltage series; analyze results of the testing comprisingcomparing the results of the test achieved using the one or morecompression algorithms using each of the selection schemes; select theone or more compression algorithms chosen using one of the selectionschemes for compressing at least a portion of the electrode voltageseries based on the analysis; obtain a compression of at least theportion of the electrode voltage series by applying the one or morecompression algorithms selected based on the analysis to at least theportion of the electrode voltage series; and store the compressionwithin the memory.
 11. A monitor according to claim 10, wherein thecharacteristics of each of the waveforms comprise one or more of afrequency of content associated with that waveform and a heart rateassociated with that waveform.
 12. A monitor according to claim 10,wherein the another selection scheme comprises setting one or morecompression criteria, applying each of the plurality of the compressionalgorithms to a part of the series of electrocardiography values,analyzing a result of compression of the part by each of the algorithmsusing the compression criteria, wherein the one or more of thecompression algorithms are set as chosen by the another selection schemebased on the analysis of the result of the compression of the part. 13.A monitor according to claim 12, wherein the compression criteriacomprise one or more of an amount of the battery amount of battery powerconsumed by one of the algorithms and an amount of memory space that isoccupied by the results of the compression of the part using one of thealgorithms.
 14. A monitor according to claim 10, further comprising:determine an availability of a result of compression of a furtherelectrode voltage series using the one or more algorithms selected usingone of the selection scheme and the another selection scheme, whereinthe testing is performed in the absence of the result of compression ofthe further electrode voltage series.
 15. A monitor according to claim10, wherein the one or more compression algorithms selected based on theanalysis comprise a plurality of the compression algorithms and theselected algorithms are serially applied to at least the portion of theelectrode voltage series.
 16. A monitor according to claim 10, themicro-controller further configured to identify noise within theelectrode voltage series, wherein the one or more compression algorithmsselected based on the analysis are not applied to at least some of theidentified noise.
 17. A monitor according to claim 16, themicro-controller further configured to represent the noise with at leastone symbol within the compression.
 18. A monitor according to claim 10,wherein the one or more compression algorithms selected based on theanalysis comprise a single one of the compression algorithms and thesingle compression algorithm is applied multiple times to at least theportion of the electrode voltage series.